Meh, don't worry about a seahorse tank. If you get harlequins you need starfish, so pretty expensive in the long run. If you have nowhere to keep regenerating the limbs of a chocolate star you'll have to make runs to your LFS for starfish to buy at ~$10 a piece and feed them to the shrimp. Neon gobys are a good idea, as are having something like a sailfin goby and its partner shrimp.
Aside from that, lots of Zoanthids, Palthoas, etc. would make an attractive display once fully covering the rocks. I'm pretty much doing this in my 24, though I do have a bit of Montipora at the very top. But yea, you can make a really neat display with zoos, and their ease of care makes them worry free.