Here's a couple examples of things I know of from other places that I have heard about...
* a wall painted with a mountain scene to inspire climbing I assume...
* one wall bricked like the outside of a building... some folks said they felt like it was riding in the city, sorta like an alley or old urban building...
* we have glow in the dark stars now, and a black light... it's like racing an adventure in the night
But then again, I have a pretty active imagination.