My banded coral shrimp is particularly entertaining (his name is Harvey but it should be King Kong as he is lord of the 140). If you get a mated pair, you can keep two but they must be purchased that way and introduced to your tank together.
Most any brittle (the red ones are very attractive and I have one that also has a personality and hence a suitable name,
Pesky ). Avoiding the green serpents is recommended and after 6 years, mine is now becoming aggressive and has to be housed separately. Other serpents, however, make great, safe cleanup crew and will come out to feed at supper time.
Flower anemones are nice too as long as you don't keep small fish (avoid a carpet anemone altogether). They are hardy, colorful and know feeding time (always an amazement to me and I have kept them for years).
Any leather coral works well in an invert tank and adds interest.
Most Caribbean sponges can be used to add color but you will want good water flow over them (blue barrel sponges are an exception and should be avoided).
Ricordeas are also hardy and do well after your tank has seasoned for a year or more. They do less well if you try to introduce them sooner.
Pin cushion urchins come in many colors and are very hardy but they will move things about in your tank. The rock urchins are less colorful (but still can be found in stripes and a couple of color combinations other than the typical plain black) and do not relocate objects. Pencil urchins are likely to eat some of the softies if algae runs short.