I've always wanted to set up a 'mock' salt water tank. As in decorate it with fake corals, etc, and fill with colorful fish and eels. Of course this wouldn't be anywhere near as beautiful as a real salt water set up, just something I would do if I had a spare tank laying around.
Personally I love planted freshwater tanks. You can look up aquascaping on youtube, you can get some brilliant ideas from there.