I'm sure others might disagree, but I'd really suggest you stay away from Starfish. I know, they can be so cool....until they die
Shrimp are definitely ok! I'd suggest a Peppermint Shrimp. That way, if you get any aiptasia (pest anemones), there's a chance it'll eat 'em.
Clams. Well, depends on the lighting in the biocube. Does it come with Power Compacts or Metal Halides? I have no idea. They can be really beautiful.
I'd throw a few snails in, too. Astreas, Ceriths....something like that. Again, others can chime in. I'm certainly no expert!