+1 on the good advice!Maybe torch frags? But don't torch corals get rather large? I'd be worried about keeping calcium and alkalinity and basically stability of any sort stable within that small of a water volume. If it was hooked up to, say, a 10-15+ gallon sump, then I'd say it'd be doable, but if it was just a 5 gallon stand alone...I'd be iffy. Soft corals would be best. This is assuming you're not planning on doing weekly tear downs and reassembles.
Fish: Trimma gobies, Eviota blennies, and basically any small fish that stays under 1". Or, go invert, and keep sexy shrimp.
CUC: Maybe a few cerith snails?
Corals: I'd really only do mushrooms and easy softies like GSP/Xenia. Unless you're highly comfortable with all the dodads and nuts and bolts and have a couple of years under your belt.
There's also CPDs and mystery snails^^ But those are freshwater things, not for a saltwater tank.