What do you normally eat on rides - what's worked on your training rides? Eat that - eat more of it, in small quantities, often. If you find solid foods to be hard to digest (not sure why protein bars should be any better than peanut butter....), make sure your bottle has some liquid nutrition in it -but find something and try it *before* your ride, as you never can tell if you will like it or if it will work for you until you try it, and in the middle of a century is a bad time to find out that your sports drink gives you terrible gas or tastes horrid or is intolerably sweet when it gets warm.

Everyone is different, so its hard to give specific advice. What works for me, may not be right for you (I, for instance, cannot tolerate bananas on the bike - they give me stomach cramps). That said, what does work for me personally has two components - it depends on how hard I'll be riding - the more strenuous the ride, the fewer solids I can tolerate. For races I rely mostly on liquids and gels. For longer more steady rides I usually take some sort of salty nutty kind of bar(s) - I particularly like Odwalla sweet and salty almond bars. Sometimes I take a little jerky - a tiny bit of protein with a wallop of salt calms my stomach and is a welcome relief from sweet, sweet, sweet. I always start with a bottle with calories and a bottle with only electrolytes.