I have fish...now what else???

I've recently set up my first sal****er tank. It's a 37 gallon with
about 15 pounds of live rock. Right now I have four small fish in it.
They are a Domino Damsel, a Valentini puffer, a Neon blue Spotted
puffer, and a Scribbled boxfish. All the fish are doing fine and get
along. I wanted to know what other kind of "animals" I could put in
the tank. Things like shrimp, crabs, urchins, anenomes, feather
dusters, starfish, etc... I understand that the puffers eat certain
crustations...Does anyone have any suggestions?

Thanks,
Phil