We usually bring
- cereal and milk (best to eat breakfast for cheap in the room rather than going out somewhere or skipping it altogether)
- whole wheat bread, peanut butter, jam (makes a nice quick lunch or dinner if you are in a hurry, and can be brought with you)
- fruit (usually fruit cups packed in 100% juice or dried fruit, since fresh fruit doesn't travel well)
- snacky things (nuts, granola, jerky, etc. travel well and will give you nutrition quickly, though we usually bring chips, haha)
- bottled water (cheaper to bring your own, much easier to carry with you than rely on convention water dispensers or finding a drinking fountain, and healthier than soda)
- utensils, plastic bowls, plastic baggies (the utensils and bowls can be washed in the sink, and the baggies are nice to carry snacks with you)
If you are relying on an ice chest, be sure to empty out the water and refill with ice often. Also try to keep as few perishables as possible so that you don't have to worry as much in case something goes wrong and your food ends up spoiled. You don't want your entire food supply to get spoiled!
I've heard of people getting good results with instant ramen noodles and the hotel coffee maker, but that's so salty that you might not want to reply on it all con (especially if you are sensitive to salt, or, as shallow as it sounds, easily bloat and are wearing revealing cosplays, haha. Be sure to drink lots of water regardless).
Typically, I'm too busy to eat lunch at cons, so I stock up on snacks and things or just skip it. It's bad, but I don't eat much anyway, haha. I typically go out for dinners when I have time (the past couple of years have had hectic Sunday evenings between the lolita fashion panel and the B&W ball, so I've just been making sandwiches and eating them on the way to the panel practice, but this year, who knows, since I'm not in the panel and it isn't even on Sunday!).
Don't try to live off of Pocky, Ramune, vending machine food, and energy drinks, but I think that's a given. It always puzzles me when people do that, since what they spend on at-con snacks could easily be spent on either nutritious and easy to eat food kept in their hotel room or on less nutritious food outside the convention center. Dealer's hall and vendeing machine snacks can be quite pricey!
Also, there's a Walgreens within walking distance and at least a Safeway within driving distance (I pretty much only know the area from going to Fanime for so long, so there could easily be more), so if you need to stock up on some emergency food or medicine, or if you want to buy your food there instead of lugging it all the way there (since I don't know where you live/how you are getting there/what your transportation situation is like), you can easily get some basics at the Walgreens (milk, snacks) or drive to the Safeway.