Then, depending on your schedule, it could be sun coral (twice a week manual feedings, or even sun corals collection, only few heads specimens to reduce pollution and physically fit into the tank), gorgonians or soft corals (preferably continuous feeding device or several times a day feeding, fine mouth-sized food, mostly zooplankton).
1. Sun coral, Tubastrea has 4 main colors (not black all have yellow tentacles): pinkish, intense orange, lemon yellow, black compact branching, black Y-shaped branches. And their relatives, same care: Dendrophyllia, Cladopsammia and others (some Dendrophillias, like firecracker, are open all day long, but Tubastreas and Cladopsammia are closed in day time, except feeding time - 1.5 hrs or so each time).
The photos are
here (photobucket is closed now for maintenance). There they are in shallow 6g, with sump for a skimmer.
The sun coral on the first photo was kept for 4-5 months in 3/4g (3L) pico with Red Sea Nano Filter and Hydor All Glass smallest submersible heater.
20W 50-50 combo PC bulb from LFS in desktop lamp for photography only, nice color. Usual daylight spiral bulb has yellowish shade.
It lived on full water changes after feeding, using water from 6g Nano Cube next to it. Both benefited from water changes

Tired after that, started to forget, it happens, then moved in tank with skimmer (90g). Now in 6g.
2. Gorgonians:
Try to find not too big, to fit in the tank. I would stay away from blueberry gorgonian (the one with forget-me-not blue polyps) and sea whips (thin, with finest polyps). Medium-high flow, frequent feeding. Orientation of the coral: across the flow.
3. Soft corals:
Scleronephthya, not dendronephthya, chili corals, and whatever else is in the LFS, Christmas tree coral (not Christmas tree rock). Same as for gorgonians, only high to very high flow and smaller food. Have no idea about Christmas tree coral, except that it usually has no problems. For a chili coral - see a separate thread below. They are doing good in some tanks, but not in the others. Also nocturnal, if the room is not bright and no lights - could be open most of the day. Same food as for gorgonians.
4. Sponges.
They don't live in my tanks. Orange-red-yellow seem do not require light. Candycane sponge looks good, polyps could be fed by Cyclop Eeze or frozen Reef Plankton, sponge - by phytoplankton. Try to find the small one, they are usually hand- and more sized.
HTH