Your best bet would be a Russian tortoise, if you INSIST on something strictly herbivorous, but even I, who often chide my fellows for suggesting outrageous container size requirements, feel that 20 gal is way too small for a tortise, except a small juvenile. Aquatics can exploit three dimensions of the tank, but terrestrials are limited to two. If you are not concerned about aesthetics and don't want to deal with a heavy glass tank, get a large relatively cheap translucent plastic storage container. These are very lightweight and easy to move and clean. Add a heat/light source, of course.