Nano fish. Lots, and lots of nano fish! Your options are limited to FOWLR, so some of the more interesting fish are out. For life, I suggest you look into:
Hippocampus zosterae-difficult-needs live food 2x daily, usually in the form of baby brine shrimp
Eviota sp. gobies-easy/medium-needs small food
Trimma sp. gobies-easy/medium-needs small food
Possum wrasses-medium/hard-needs copepods, very shy
Small shrimp gobies and Alpheus randalli shrimp-easy/medium-needs DSB
Elacatinus sp. gobies-easy/medium-needs small food
Firefish: Nematoleotris sp.-easy/medium-shy, you could only keep one.
Apogon parvulus-medium-needs a school of at least 5.
Barnacle blennies-medium-needs small food.
CUC: Stomatellid snails if you can find them (10), Astrea snails (2), hermit crabs (3), lots of copepods, amphipods, bristleworms, munnid isopods, etc.
Equipment needed: A heater to keep the fish warm at 78F/25.56C. A protein skimmer to clean the water of proteins, one rated for 15 gallons/60L + preferably. A double fluorescent light fixture or similar LED, PC, whatever fixture to see your fish (one 10000K bulb, one actinic (460nm) bulb). A small circulation pump to allow for water movement (at least 100gph). A good salt mix, like Reef Crystals, or just plain Instant Ocean. A refractometer/hydrometer to measure salinity. A thermometer to make sure you aren't freezing/boiling your fish to death. (Optional) Automatic Top-Off (ATO) which cuts down on you having to constantly top off with RO/DI water. (Optional) RO/DI filter to make replacement water (*if you don't use RO/DI, you'll probably get algae problems later on). Ammonia drop test, nitrite drop test, nitrate drop test, and pH drop test as well for you to keep up with the water quality.
Substrate: depending on what your fish will be, you'll probably want a very shallow sand bed. Around half an inch (2.5 cm) to an inch (5 cm) deep, of oolitic sand would work. 15 pounds of live rock, or 10 lbs base and 5 pounds live rock would be good as well (it's your main form of filtration). Unless you go with the pistol shrimp, then you'll need a 3 inch (15 cm) deep sandbed of mixed material for the shrimp to make its burrow.
Food: One dry food (flake/pellet), such as Omega One Marine Micro pellets. One frozen food, such as cyclop-eeze, Calanus, etc. If you really want to spoil them, give them the prawn roe that you sometimes get with shell-on shrimp, or the fish roe if you catch saltwater fish (similarly, flying fish roe that is used in sushi, can be bought at most asian stores).
Additional stuff that I need to ask: have you ever kept fish before?
