I'm a fairly high-metabolism type of person, and if I don't eat breakfast I really feel it even if I don't do any exercise. Unfortunately, I tend not to be very hungry right when I wake up, so it's a tough one to reconcile.

I've found that protein/fruit smoothies go down pretty easily even when I have to force it down, and they prevent the mid-morning crash that I would get otherwise. If I have a bit more time to wake up before breakfast (like an hour), then I like yogurt+granola+banana, sometimes with a bit of peanut butter if I really need nutrition (like before a race).

That being said, I've found that biking is much easier on the stomach than running, and as long as I'm not doing hard intervals or something, I can pretty much eat whatever and feel fine.