Turtles of the slider variety are nice but do require a lot of work. I had a pair for about 15 years - moved from a 5 gal to a 20 gal to a 65 gal tank over that time (got them when they were the size of a quarter, final size was 6in across and 9in long). The keep the smell down you have to clean clean and clean some more.
A collection of hermit crabs would do well in a small tank. You could make your own little hermit city.