@Oceanarium has paper snails, mini brittle starfish, mysis
Hit
@NiCd up and see if he has any clean up crew for sale at the mo - turbo snails, ceriths etc.
@Dr. Schell may have some things for you as well such as dog whelks
It's worth asking around :)
From LFS if they have them: I'm quite partial to dwarf yellow tipped hermit crabs. They look like this:
I find the light brown microhermits hard to keep personally and some of the bigger types are a nuisance. Whereas these are sweet little things that will grab some of the food when you are feeding the tank. Some hermits will overgraze your rocks but I find these guys good. I had two in a tank your size.
A saron or peppermint shrimp would make a nice addition as well.