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?
white biorb spanish dancer sculpture also looks backwards set up in aquarium to me
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.