If you plan on decorating your tank with fish (not the other way around, providing decorations that a particular type of fish might find useful), then consider whether or not your decorations are functional. For example, something like those divers that hold up "NO FISHING!" signs are pretty useless, except maybe for fish who like to eat algae, since just about anything in the tank accumulates algae. Caves and things of the same sort (pieces that have holes and hideouts in them) are functional. A lot of fish like hideouts, and as long as they are safe, it's up to you if you want a castle or a cave or a sunken ship, a fish indeed could not care less. If you use fake plants, keep in mind that silk is safer than plastic for fish--safer for their fins and bodies. Also, many bottom dwellers would *prefer* to have sand, not gravel, so if you have gravel, steer clear of fish like kuhli loaches, their gentle bellies like gentle substrate.
Post a picture of your tank, too! It would inspire a lot of suggestions, I think.