I discovered after my first convention that oranges are the best food you can have during a convention. They're great for breakfast, especially since I get queasy when I eat grain in the morning, and apparently Vitamin C protects against sickness. They also don't need refrigeration, so it's perfect. I have an orange every morning of a convention.
The other two meals are iffy. Sometimes I eat, sometimes I don't. Friday morning I make some rice balls to eat that afternoon, but the rest of the days are complete hit or miss. It's hard to remember to eat when you have all these awesome things to do and see.