Just about anything that can be BBQ'd. For larger groups, we often do speghetti, chili, or other things that cook well in a pot.
For breakfast, if you like eggs, you can crack them into a tupperware at home, then put the tupperware in your cooler. The eggs will stay together, and you can pour them out singly or as many as you like, with no danger of big messes or shells in your food.
A good breakfast item is an item that seems to have a hundred names (mom called them "one-eyed gypsies", but they're also known as "frogs-in-a-hole", "boxed-eggs", and many other names). Basically, you take a piece of bread, and use a cup like a cookie cutter to chop out a round hole in the middle of the bread. Then, put the bread in a buttered frying pan and fill the hole with an egg. Don't forget to toast the circle of bread too. You cook it just like a fried egg. Mmmm good.
Camping, of course, requires many snacks, including Red Vines, pretzels, beef jerky, M&Ms, and plenty of fruit. Watermelon cooled in a stream is great when camping...
-Troy