What type of substrate do you have? A pistol shrimp/watchman goby pair are great in a tank with crushed coral or a coarse sand substrate. Fine sand substrates may need a fed more coarse pieces for the shrimp to work with in building its tunnel.
Also, if you get a pistol shrimp without a goby, you won't see it out much. They are much more active/visible with a goby partner.
Brittle stars can be good additions as well, but avoid the green serpent stars, as they tend to be fish eaters.
Kevin