Some smaller geckos like leopards would be cool. Or if you want to keep fish, go with a group of (most) hypancistrus Genus Plecos and breed them. Most babies sell for about 30 bucks each. With some Like L399 going up to 80-100, and zebras selling for 250.
Here is a genus list.
http://www.planetcatfish.com/catelog/genus.php?genus_id=7
Easy to find are L260, L262, L136, L316, L333, L287, L066, and if you have the cash, H. Zebra.
If you go with the larger hyps like L333 you can only fit 3 in a 20L but 6 of the smaller ones like H. zebra or L136.
If you want, PM me and I'll get you a list of breeders, I can also help breeding them and tell you who is male/female.
I've written several "How to" on these guys.
Theres also many other small interesting non pleco species I can recommend. PM if you want a list.