If I don't eat three meals a day I get lightheaded and lethargic and I don't retain water (bad when riding).

If I'm doing heavy physical activity for more than a couple hours, I need snacks as well, but more than a small portion slows me down. As for hydration, I like to make lemonade with salt to put in my water bottle. That and a bannana is probably the best method short of sports drinks for going for more than 2 hours at a stretch.