I get the "What DO you eat?" question all the time (second only to the ever-popular "How do you get protein?"). I also stumble over finding an answer, since I eat a pretty infinite amount of different foods. Usually I'll just cover the food groups and tell them that I eat lots of fruits and vegetables, pastas, breads, grains, beans, seeds, legumes, and anything else that doesn't come from an animal. If I don't feel like being too detailed, I'll just say "lots of things."