I go with careful planning, cargo pants (wallet, phone, phone battery, water, water) and a blackrapid camera strap (big camera). Sometimes I have a lens pouch (70-200mm lens) on my belt. Oh, and my badgeholder (room key, spork, chopsticks, protein bar, pens).
If I have an event that requires a game book etc, I guess I'd resort to a bag, but I'm not at gencon for that kind of gaming experience. I might have folded up paper in a pocket, but if I play an RPG, I'm either a total noob at the system and a book won't help, or its a game I know well and I can check details on my phone or ask the GM.
The single most important bit of gencon gear is good shoes (clean socks (feet)). The amount of walking involved in a gencon for me makes this vital, and is why I am for a light load. I have gone so far as to buy new shoes at circle mall during the convention when the ones I was wearing were just not working out. If I were to carry a big backpack and lots of stuff, my back would cut my day short.