Patience and research are the keys to success! I would recommend xenias, mushrooms, kenya trees, leather corals, and (when you get into LPS) candy canes! Of course you wouldn't have to go in this order, as long as your tank is mature and stable you can keep anything!
As for symbiotic relationships, my favorite is the yellow watchman and tiger pistol shrimp. Mine paired up within 5 seconds of introduction and went digging away to make their new home. Very cool behavior (only downside is that the shrimp works so hard that if you have sand, it'll be all over the place).