Urchins can be a pain. They're bulldozers, moving around at night knocking things around. We have a couple in different tanks, and both came in as hitch hikers. If you don't mind the occassional re-arranging, you can get one, but they aren't really that exciting. During the day, they tend to hide (and can fit in much smaller holes than expected), and then scrounge around in the day. They can be a threat to some corals (reportedly, they can poke them and damage them), but we never had that problem.
You could look into crabs. Sally light foot crabs are a favorite of mine--they are a bit shy, but pretty, and very hardy IME. We have 2 in our 65. They fun to watch, especially during feeding time, when they going jumping around after food. Peaceful, get along with most other inverts.
Also, check out the variety of snails. Nassarius, cerith, turbo...Bunches out there. Be careful though--I've seen more and more coldwater snails being offered up. They don't do well in a tropical tank, lasting maybe 2-3 weeks.
Conch are also an option. The fighting conch is readily available and stays fairly small--one would be okay in a 30.
Scallops are fun--we really enjoyed our flame scallop. He wasn't ahppy with us, and moved around too much--got too close to a coral that stung it to death. But, they are neat to have. I'd stay away from clams-you don't have enough light to support one. Filter feeders require special attention to thrive, so getting one for a new tank that needs high light as well is asking for trouble.
Just to make sure, have you planned on doing a fishless cycle of the tank? Inverts tend to be sensitive to water conditions, more so that fish. You won't want to add the inverts until the tank has cycled, and I recommend cycling fishlessly--throw in a chunk of cocktail shrimp from the deli to kick start your ammonia, and then test. Ammonia and nitrites should be zero before adding any inverts.