You are limited to the tangs you can keep in that size tank. Make sure to do your research before buying one.
I would suggest wrasses. Most wrasses are reef safe (there are some that are not) and they are smaller fish, so less of a bio-load. They also have amazing color diversity and varying temperaments.
Liveaquaria is always a good place to do some research on fish, they will give you the information you need to make good choices. Sometimes their tank size recommendations are a bit on the conservative side, but most other places will give you nothing close to the right size for tanks, so I usually err on the side of caution with Liveaquaria's recommendations.