About the only starfish that are appropriate for a 12 gallon tank are the little white asterina stars, and the little mini brittle stars. Mini brittle stars are great for a clean up crew and they always have their little legs peaking out of the crevices. They won't add tons of color though and are very small. Anything larger will starve and die.
I think all of the coral suggestions so far are really good. Definitely give some xenia a shot. It is about as hard to grow as dandelions in most tanks and its motion is a great addition to nearly any tank. For color I'm kind of partial to the riccordias, but there are tons of very brightly colored zoas out there as well.
Skunk cleaner shrimp, fire cleaner shrimp, or pederson cleaner shrimp are all great options. They will add color and motion to a tank. A coral banded is a nice addition as well, but it will kill most other shrimp including other coral bandeds unless they are a mated pair.
The colored feather dusters are great as well. They are easy to care for and nice looking. Everyone seems to notice them right away as they are familiar to most people too.
