depends on what you like and what effect you're going for. wood, stones, or plants in any combination is my preference. I've never really gotten into the castles and gone fishing signs.
I think making the tank look like the hold of a sunken ship would be cool, but a lot of work.
well it all depends on the fish u plan on getting u should just replica there natural invironment thats alwas the best thing for the fish plus lots of plants