plants (live or plastic), a couple pieces of slate, driftwood, and some various rocks will always be nice additions for hiding places. some rocks have holes cut in them and are fun to watch fish swim through ... or if they don't swim through then u might be able to see the fish through them. slate is my favorite for making caves. propping driftwood on a rock or two will also make a swim-through shelter.
HTH and have fun (aquascaping is my favorite part of fish keeping)
edit: sorry for repeating - we musta been posting at the same time, cheese