Don't do any coral or dead sea creatures. They will change the chemistry in your tank. If you want to go that route get resin copies.
I honestly don't know what you can decorate with, it depends on what you want.
black biorb pebble sculpture (they put it backwards in video though?
http://www.youtube.com/watch?v=LXfryUT5Qns
white biorb spanish dancer sculpture also looks backwards set up in aquarium to me
http://www.youtube.com/watch?v=Bei-Rxv5HRc
looks like that company has a lot of black and white thingies, you might be able to find plastic plants in black and white?
I don't know if art glass is safe for aquariums but if it is (make 500% SURE it won't leach lead into the water.) that would open up a lot of things you could stick in there.