Hay rock, if you can add some power compact light to your tank you could keep some mushrooms, colt, toadstool leather easy corals like that. Do some reasearch to find out what each coral requires first(light, water movment, feeding) than decide what will work in your setup. If you aren't sure about certain corals, than post questions on the reef board.

If you don't want to upgrade your lighting, and want to add some cool stuff, i suggest adding a cleaner shrimp an/or a brittle star. These are both interesting to watch.
Is your cycle all done in your tank? If not, thanyou will have to wait to add any inverts. They are very sesitive to water quality. How long has it been set up? What are your water tests reading, ammonia, nitrite, nitrate, pH, alk?