I would suggest snails. Apple snails are in almost every petstore or you can buy the really nice blue, ivory, and yellow ones online. You can put as many snails as you want to in your tank, if youre worried about breeding thats EASY....apple snails lay their eggs above water, they are quite easy to spot, so you dont want more snails just flush the eggs plain and simple. They take about 2-3 weeks to hatch so you need only check once a week or so.
There are also a wide variety of smaller shrimps (ghost, red crystal, etc) I would stick with the SMALL ones versus the large ones - or even crayfish- in a tank with fish in it, because the larger ones can and will eat slower fish.
You can also buy aquatic frogs. The african dwarf frogs stay small and are definately funny to watch.