I've been going to the Con only about 5 years now, and my first year 2 of us took big expanding back packs crammed full of everything we could think of. Each year our packs got smaller and smaller and now we just carry 1 messenger bag with a couple bottles of water, mints and a travel mouthwash, travel deodorant, a few snack bars, and headache pills. We do keep a cooler for lunch stuff in the car as well as a change of socks each day and extra pair of shoes if we're wearing sandals, plus some miscellaneous clothing changes for the weather.
We've found it better to carry less and if we do need a bag for something extra we've usually made a purchase and have at least one extra bag.
We've found that the badge wallets are great for miscellaneous pencils and small items as well.

Plus the messenger bag strap gives the smaller ladies in our group something to hold on to while the bigger guys make a path in the dealer hall.  Nothing like turning around and finding out you're missing someone only to see a familiar hand jumping up and down 2 intersections back. LOL

• Hand Sanitizer
  
• Portable usb charger
  
• Cell phone
  
• Tickets
  
• First Aid Kit
  
• Pens
  
• Tissues
  
• Chapstick
  
• Compact mirror
  
• Pair of travel scissors
  
• Fabric Adhesive tape role
  
• Fabric Glue (I'm a cosplayer and I always lose my needles if I try to bring needle and thread)
  
• Some snacks
  
• Water bottle
  
• Bottle of Bubbles
  
• Dice bag
  

Silly string

For me its gonna be
A) Dice bag
B) character folder
C) Folders for events i will be running that day
D) Tablet (with all my stuff in word doc form)
E) keypad for use with tablet
F) extra pens/pencils.

I wear Cargo Pants and keep...

Dice, Pencils, Hand Sanitizer, First Aid Kit, Miniatures First Aid Kit, Daily Meds, Glucose Meter, Bag of Beef Jerky, Breath Mints, Etc in the various pockets. And usually have an army surplus canteen on a belt for hydration.

I do carry an old Canvas Old Navy Army Surplus style bag with whichever miniature game I'm headed to next. But that's all that goes in the bag. And I can often leave it in the room/car for most of a day and go with just the stuff in the cargo pockets. It makes navigating a lot easier.

1st time at gen con but not my first time at a convention/festival so here's my list-

Small crossbody bag containing:
Phone 
Baby Wipes
Hand Sani
Cash
Plastic
ID
Water

That's it. 

Airborne is junk (IMO), just take a vitamin C (or 2) with breakfast every morning and you should be good. I used to get colds a lot until I started taking C right when I felt one coming on and I haven't been sick in years now. It's so insanely simple I kick myself for not taking better care of myself sooner.

Last year I carried the Thinkgeek "Con Survival" bag and it was fantastic. I start the day with it carrying:

1 Bottle of Water (I refill it throughout the day in the halls)
1 Phone Charger (Rapid, 2.4 amps)
1 Small bottle of ibuprofen (Just in case)
1 Program Guide
1 bag of dice 
2 mechanical pencils
Random Snacks 
Hand Sanitizer

- Gaming folio (character sheets, notes, pens, notebook)
- Meds
- Hand Sanitizer
- Dice
- Power bank or 2
- Charger cords
- Snacks
- Tablet
- Cane (since my bod has been painful and dizzy, which is not a good combo for a crowded dealer hall)

Trying to pack light this year. I always over-pack, and I never need half the stuff I take.
Taking my tablet to eliminate the need to haul any rule books.  Plus, I'll be in the Catalyst room playing Shadowrun all day, and hubs will be there running stuff, so chances are he will have books if I need them for some reason.

It makes me happy to see so many people mention writing utensils, as in the real world I feel like I'm the only person who ever has a pen and people are always stealing mine. One thing I am surprised nobody has mentioned is sunscreen. I know some people will be staying inside pretty much the entire time, but it's the middle of August and I saw a lot of people turning pink outside in the afternoon waiting for food, walking downtown, socializing at the beer garden, etc. I try to pack light, but I bring a bag at least big enough to fit the Program Guide in. It usually contains:

- Program Guide
- Pen/pencil/sharpie/highlighter
- Small pad of paper
- Phone
- Charging cord and battery
- Sunscreen stick
- Chapstick
- Multitool or small knife (prevents packaging rage)
- My agenda
- Washcloth
- Dice

I do not carry a water bottle since there are plenty of free water sources around the convention center/hotels and even an empty bottle takes up a decent amount of volume. Other things I make a point to have that don't necessarily go "in the bag" are cash and a watch.  

Couple pens, couple pencils
Small pad of paper
Pad of post-it notes (for keeping track of hit points, etc.)
USB battery bank & cord
Dice (my travelling dice bag, less than fist-sized)
Snacks (I'm a local, so I pack enough snacks for the entire day)
Water bottle
Tiny first aid kit with Ibuprofen, Tylenol, Imodium, Tums, a couple safety pins and a couple band-aids
A few paper napkins
Nail clipper and emery board
Folded up bag for overflow purchases (one of the shoulder bags they gave out as swag a couple years ago)
The coupon book
My auction bidding card

Not in the bag but always on me:
Badge holder with badge, pen and pencil, tickets and printed schedule
Phone (with the unofficial Gencon app, which I basically just use for the maps)
Watch
Money, ID, credit card, insurance cards

I no longer carry rulebooks or the program since I found I never really need them. My shoulders are much happier now!

I usually just bring my purse which has most of the small essentials in it already.

However, this year I bought a backpack purse just for the occasion so I can convert it between being worn as a backpack or a shoulder bag. In it I plan to pack:

event schedule, print outs of the convention hall floor plan, parking info, hotel info, exhibit hall pdf
wallet 
cell phone
hand sanitizer
Aleve
a water bottle
a small bottle of contact lens solution and a spare case 
hair ties
small personal care kit
snacks

In my badge wallet I will keep my event tickets and some cash

In the car I will keep a change of clothes and extra pair of shoes, as well as a cooler with ice cold water and Pepsis

Fethbone, I was initially so confused that I was going to lightly ridicule you for suggesting sunscreen, but I grant now that food trucks are such a highly-recommended part of Gen-Con that people really do spend time standing around outside.  It just so heavily clashes with my vision of Gen-Con life as a 99% indoor activity, haha.

Also TDB, wow, packing your Auction bidder card even before the event starts?  That is prepared!  I doubt they will let you use last year's card, though; I think I saw someone try that once ;)

Two must have items

1.) Hand sanitizer
2.) Small pack of tissue/toilet paper (the convention staff cannot keep up with the bathroom maintenance)
 

Chrome Messenger bag for casual/gaming day:It has lots of pouches/zip-up sections...

iPad: It has PDFs of all the D&D,TOR,PF& Fantasy Age that I might play, and (Shocking) none of it is pirated! :D.
Charger/battery for ipad/iphone
Dice bag with only the bare-bones number of Q-Workshop dice (OK, about 30).
13various colored  Micron pens, as I sketch & jot down notes old-school style here& there.
Folder containing my current D&D characters for pickup games
Snacks (Turkey jerky, trail mix, some dark chocolate truffles from the excellent chocolate shop in Monument Square), Cold brew iced tea bags
Nalgene bottle, for water/tea.
Gout/Asthma meds + pepto,ibuprofin & tissues,plus small hand sanitizer.
Extra socks... I walk a lot, and fresh socks after 8 hrs of adventuring is Heaven; Make sure you change them away from the game table...hehe.
Also, deodorant.
A flask of...Um, Nvm.

<If I'm planning on running something>
Dozen or so ArcKnight tiles, bag of pre-paint minis, dry erase markers. Character sheets.

On Thursday, I plan on staying in the dealer hall until it closes, getting as much shopping done as possible, so I will switch out the messenger for my Gigantor backpack. Fear not, as I work in a nightclub, and have excellent and courteous spacial awareness regarding whacking folks with my bag...

Cruising around to dinner, etc...small side-satchel, with maybe a small boardgame for pick up fun, sun screen.

I'm still debating on taking my backpack, or getting one of those ThinkGeek Bags of Holding - Con Edition, but these ideas are all great. 

It's my first time, so I'm also thinking about bringing my Nintendo Switch? Not sure if that'd be a waste or not. 

I've learned after going to Adobe MAX a few times that good shoes and drinking water were some of the most important things...and maybe a hoodie as the conference centers can sometimes get a little chilly. 

I actually thought the ICC staff did a much better job last year than the previous years with the bathroom upkeep.  That said, you should probably avoid the bathrooms near the card halls if at all possible anyway, but generally speaking it wasn't as bad as it had been in the past.

A key thing I always include in my con bag: Empty space. I know I'll fill it during the day with business cards, little things I simply HAD to have, extra water/soda bottles, etc. I've made the mistake before of leaving for the day with a full bag, and it never ends well.

Last year, I used the Equilibrium USG bag that I purchased from the 2015 con. It distributes the weight pretty well, but the bag portions I have are pretty big (which I don't want to use on the floor). I will be thinking of using a smaller bag that could be used as a messenger or backpack as well as hook up to my camera's BlackRapid strap.

The lesson I learned from my first year is that I want good shoulder straps and good weight distribution (narrow straps digging into your shoulders for 8+ hours is quite uncomfortable).

One other thing that I will do (that I've done from travel) is to use either locks or wires to keep the bag shut (I haven't run into issues at Gen Con with stuff disappearing from my bag, but it's a habit I've had from other travel).

Inside of it will be a lot of the usual stuff people have mentioned:

• Gaming tools (dice, paper/index cards/notebook, pencil/pen)
  
• Insulated water bottle (using the on site filling stations; note the filling stations are hooked up to the building water source, but isn't that cold)
  
• Portable battery and phone cable
  
• Sunglasses (useful for my longer walk to the ICC and while eating at the food trucks)
  
• Small sanitizer
  
• Small snack (protein bars/trail mix)
  
• Ibuprofen/Excedrin
  
• Earphones (in case I need to block out noise and recharge)
  
• Lip balm
  
• Contact solution
  
• Camera (and spare battery)
  
• Small/foldable fan
  
• Bandana/microfiber hand towel/small cloth
  
• Couple of safety pins (in case of clothing rips/emergency help for cosplayers)