There are a ton of things you can get for that tank and light setup.
Sponges, gorgonians, sun corals, sea squirts, etc. HOWEVER all will require target feeding and that can be real bad for your water quality in a tank that small. Most of these guys don't have any light requirements at all, so, if you go this route, I would only get one of them and go easy on the feedings and see how things work out. I would go with a sponge first, as they (for me) were easier to care for than any of the others I mentioned.
Second, I would do what I did in my 100 gallon medium light tank - fill it with mushrooms. I have metalic green frilly, lime green, red, purple and blue mushrooms in there, and they really do look good. I had some zoos as well, but my juvi emperor angel decided he liked to snack on them, 'nuff said.
I had a small scroll coral frag (SPS
) that did OK stuck up in the uppermost few inches of the tank, but it was not doing outstanding. I was not dieing, but it was not growing any. Kenya tree coral did very well in my tank as well until my angel sampled that one day as well. A mushroom leather was also fine under that level of light as well.
So, I would generally avoid the hard corals but there are a few that could work for you. Nearly all of the softies will be fine (depending on placement) under 3.5 wats per gallon.