Haven't done this in years, but many good resistance exercises can be done on the road with thick rubber tubing, of the type that can be bought at pharmacies (surgical tubing) and hardware. You can also buy a fancy version with handles at some fitness/sporting good stores, but I never bothered.
I can't say it's going to build you mighty muscles, but it's a workout that you can feel, if done properly. You can make it as hard as you want to, by buying thicker-gauge tubing.
Here are a couple of links that go into more info:
http://www.yorku.ca/wellness/levy/tubing.htm
http://umanitoba.fitdv.com/new/artic...html?artid=405
There are even books. Google "rubber tube workout" and you'll find a few.