Frogfish tank.
Bonsai acropora millepora tank. One small outcrop of rock with an acro colony on it, pruned for shape.
Xenia only tank. Let the weed go wild.
Carribean flower anemone garden, with lots of Sexy Shrimp.
Pizza anemone & domino damsels tank. Surprisingly effective display.
Cerianthid anemone species tank - heavy actinics and maybe even add a blacklight.
Conch fish species tank. Get a bunch of empty conch shells and buy a handful of the cardinal fish that lives commensually in them. It'll use even the empty shells, though it would prefer live conch. Use subdued lighting.
Vivarium. Dan Conner's the expert at this, check out his stuff. I'd vote for keeping pearlscale killifish in this, personally.
Banggai tank. Get wood pilings, seal them to keep them from leaching, then build dock pilings with them. Try to make it look like you're under a dock. Primarily keep a pair of banggais and some longspine urchins, but fill it out with other appropriate organisms for the habitat.