I wish I could post a picture of DDs room....she has a fairy room which makes jaws drop. I'm only bragging because it's so unlike me to do something 'cute' :) I found a website which sells scene murals. They run anywhere from $60-150 depending on size. They usually take up a wall to a wall and 1/2.
For her, I put up a mural called "Storybook Hollow" and painted the rest of the room in a chair rail style, Tinkerbell green below and Tinkerbell purple above with jewels for the chair rail. I put up a few stickers of Tinkerbell, found at homedepot, on the mural and in the room. Her bed is just a normal girl's bed and I don't have special bedding, just what was on sale at bed, bath and beyond.
It looks like a Fairytale land and the whole thing was under $200!
For a boy, dinosaurs, space -hang planets from the ceiling, space mural, cars, you name it.
have fun!!