As a convention staff member, I know how important it is to eat properly and stay hydrated. It has happened more that once that a person fainted because of dehydration or malnutrition. There are a few things you can do to prevent this from happening.
First of all, most conventions are hosted at hotels. They have their own catering, which serves drinks. If you make use of this service make sure you DON'T buy softdrinks like orange soda, cola or anything else sweet. Ask for drinks that contain lemon or other fruits (real juice!) or bottled water. They contain more nutritional elements then soda's. Energy drinks may seem like a good idea, but only drinks like Aquarious and Gatorade make sense here, since they rehydrate your body effectively.
Bringing stuff will not make the catering staff happy, but have in 0,5 liter (about 0.3 OZ) bottle of water with you at all time is a good idea. Tap water will be readily available but don't overuse it. In some countries drinking this is a BAD idea, thus stick to the aforementioned bottled water, Gatorade and Aquarious.
Now, onto the food. You need three meals a day, no exceptions. Why? To avoid con flu. What now, con flu? Yes, you're in a crowded place, the ac is cranked to max, not everyone is taking care of themselves right and sickness is abundant. You need to arm your body with proper nutrients! To get those, you must eat. But you also have to eat right!
First off, junk food is a firm no-go for weekend conventions. For single day events they can be okay. Get a proper meal at dinner and lunch, try to get a LARGE breakfast. A large breakfast will provide you with a solid basis. In most hotels it's included in the room and an English Breakfast (which I personally prefer). It gets you heaps of protein, iron, vitamins, nitrate and other nutrients. Skip yogurt, cereal and stick to bread, bacon, scrambled eggs, coffee and milk. For lunch, try something with fruit or a nice sandwich to top off your breakfast. For dinner, have yourself a good meal, stay away from foods with too much fat in them. Pasta and (fresh) noodles are good to go. Salads are even better and try to get fruit for desert.
Lastly, snacks! Chocolate snack are great for an energy boost, fruit gums with vitamin c are good supplemental and cookies can't be beat because the hive fiber and sugar. Try chocolate chip cookies! Snack can be a good ice breaker as well, but we'll talk about that in a later entry.