how stable are your levels? some type of SPS like a montipora or a clam would be awsome. My tank has been set up for almost 9. i added my first SPS almost two months ago, and i added my first clam about 2 weeks ago. So far everything is going well. However, there are more things that youw ould need to take into account if you are going to keep these besides water quality and lighting. Not something you want to rush into. If you want something hardy like you said, neither of these would be a good choice, but i thought id throw them out there if you would like to do reserch and get more info for the future.
but maybe you can add a red open brain. I have one in my 40 and its doing great. adds some nice color.
or a frogspawn or hammer. These are my two personal favorite LPS. They can both come in florecent green, cream, and i have even seen a hammer that was blue.
maybe a bubble cora? another LPS. They looks realy nice and flow a little bit. They come in white, pearl, or green.
a blasto meretti or welsi would be realy nice. I absolutly love blastos. But they are freakin expensive. It depends on how much you want to spend. They come in a huge variety of colors too.
and then there are the ever poplular mushrooms!n You can buy them in any color you can think of jsut about. Any combination. And i happen to love them. They are a lower lighting coral though so you will have to keep them to the bottom and the side of your tank. I have some under my halides and the colors are realy nice even though they are all the way over and at the botom.
The mushrooms are the hardiest of the ones listed above. All should do great with proper water quality. All the oens i mentioned are moderate lighting however. The frogspawn and the open brain will probably do better than the otherones with brighter conditions. Also, all require medium flow.