My boys' room is a forest animal theme. The bedding has squirrels, raccoons, owls, foxes, bears, etc and the sheets have pine trees, acorns, and animal paw prints. We have stuffed animals that decorate the beds when they are made that are the same type as on the bedding. Over each bed is a raw wood plaque with bark edges that I painted their names on. The walls have framed pictures of the same woodland animals, with baby animals on one side of the room and the adult version on the other side. This was the easiest part because I just did google image searches for "baby skunk" "owl" etc, printed out the pictures and then bought $2 wooden frames at Wal-Mart. They look really cute.
The bedding I also chose to buy because it is reversible so if any of my boys decides they don't like it or outgrow it, they can flip it and it will still match the room.
I like the pp idea about doing a color scheme and adding accessories. It would be super easy to find a red or blue bedding set, toss a stuffed mickey and goofy on the bed, and frame some pictures of the characters on the wall. You could probably find a Disney calendar and frame the pictures out of that! Then he could have his Disney room, but nothing is set in stone 
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