What kind of lights do you have? This will make a big difference in what corals you can have.
Some to look at: mushrooms. Sort of the guppy of corals, shrooms come in a variety of colors, are fairly low maintenance, and reproduce easily. The problem is they can be very aggressive, and kill neighbors. In good conditions, they can reproduce very fast, and take over a tank. I'm a big fan of the fuzzy shrooms--love 'em! But they do appreciate higher light levels than the common green and purple varieties.
Leathers: Many kinds. Toadstools are pretty hardy, and attractive, if you find a good specimen.
Polyps. There are tons of kinds of polyps. Many will spread easily, some to the point of being a pest.