Many people recomend one snail per gallon. If you keep hermit crabs, expect to replace a few of those every now and then. Hermit crabs are useful scavengers, I'd keep a couple even though they pose a risk to the snails. A shrimp (or pair if they're the type that won't get territorial) and a emerald or proclain crab should round off your tank. (The Emerald will eat algae, I highly recomend him if you're not using RO/DI water. The Proclain is a filter feeder, and will need supplimental feeding if you don't have enough "pods" in your tank.)
Just my idea, there's a wide range of inverts out there, like fan worms, corals, urchins, cucumbers, scallops, clams, nudibranchs and starfish. I'm just recomending the less troublesome ones for starters.