We moved our son into his "big boy" room when he was about 2.5 (he'd been in the nursery until then).
We picked most of the stuff, but we're continually changing things out (wall decorations, etc) based upon his changing desires. It's been pretty consistent for a while. The good thing is, at that age, unless you go with a movie character theme, I think you have a lot of flexibility without spending a lot of money.
He has a shelf full of books in his room (it's actually a cubby system that also has baskets to put toys into).
We also put his library books on the shelf on his nightstand so we can keep them separate and tidy. He's really good about keeping thing in order.
The only thing we WILL NOT put in his room is a TV. He's a smart little devil and would have it on all the time. My husband's an avid reader, so I'd rather his room be void of too many distractions other than those that spark his creativity.