Zoas are cool and don't need a whole lot of light (those were my first corals 13 years ago and they are still going strong)...and they are not invasive as some others are such as mushrooms. Xenia is really beautiful but can become a plaque in the tank.
I find any of the leather corals easy to grow as well. Star polyps are also cool as space fillers. All of these are pretty easy to keep and grow and won't get in the way of other corals when you want to try some LPS corals.
You could try a small frag of frogspawn, torch or galaxia LPS corals. I find those as easy to keep as some of the soft corals and they won't break the bank if you start small. I personally think your light over a 55 gallon should be plenty for a lot of different corals.
So, reading back, exactly what Saltanklife said...