A percula clown would do well in that tank
, mushroom corals don't require a huge amount of light, and like suggested before, shrimp can be a nice addition. Featherdusters don't require any special lighting either and can make a very nice display!
Almost forgot....I run a filter on all my tanks, even my little 5g. Why? Because it's a good place to run some carbon and/or other material (I also use Phos-zorb), it will get some of the detrius out of the water column, and it will also provide current. You just have to clean it regularly.