Aquarium vert/inverts (other than fish, snails, and /shrimp/crayfish)?

I want something for my aquarium that is different than just fish and shrimp. I'm getting snails to control algae since I put in some ferts, but is there a cool vertebrate or invertebrate that will live in the aquarium with the fish. I have had African dwarf frogs, too. Any ideas? I know it's very very limited. Thanks!