It's a tough call, what does your system looks like, or even what it will look like in the future...
I've always liked the sea cucumbers but since I've removed my sand bed, I don't think one would do to well in my tank
Do you have sand bed?
-maybe nassarius snails, butterscotch snails and hermit crabs, or maybe a sand sifting starfish.
Are you looking for detritivores
- maybe mini- stars, cerith perhaps fighting conch
Is algae an issues
Hermit and Emerald crabs
Ultimately after you have done your research, what do you have questions on.
Also, something else I saw that Shac ( Reef Shac) has posted a CUC Sale on his sponsor page.