Right now, we are planning on giving my son a " big boy bed" and decorating his room in a Spider Man theme. My husband is a cartoonist so he's going to draw on the walls. I'm going to make the curtains and probably sell a bunch of things on eBay to pay for sheets and a blanket.
Right now he is into "planes, trains, and automobiles", so I plan to paint clouds on the light blue walls and hang kites and airplanes from the ceiling (I haven't done it yet, but I have all the supplies to do it soon). I have made some new artwork. I can't paint cars, but I painted scenery, then carefully cut out photos of his grandparents kayaking, DH on his motorcycle, and his other grandparents off-roading in their Jeep and added them to the painting. I think the result is really cool and DS was excited when I framed them (I'll try to add a photo link when I get a chance). I sewed shear curtains with fabric that is a blue sky with puffy white clouds. I also found some "cloud" sheets at the thrift store for a couple bucks. Eventually we will convert his toddler bed into a full-size bed...but this may be a couple months away. I've got to save up for a mattress first.