Actually, all kidding aside you could hold and release all your muscle groups, abs, thighs, calves, glutes, arms....It's not excercise but releasing and contracting your muscles is good...
you laugh, but I do my kegels (however you spell it) on the way to work and on the way home. It may be TMI, but i've been doing that for close to a year and don't feel like I have to worry about an "accident" if I drink too much water...lol
If you're not driving you can chair dance to the music on the radio, flex and point your toes, do those isometrics mentioned above, just tightening your stomach muscles every so often helps.
I've thought of this too just because I commute for about 2-3 hours everyday and I think, what a waste of perfectly good exercise time!....unfortuantely, I am not riding, I am driving so it's too dangerous. I was thinking of trying ab-work while driving just by holding in my tummy and twisting lightly occasionally. Also rush-hour snail-paced driving I can get a few arm stretches in but I tend to get funnny looks because I'm usually singing at the top of my lungs too.