In anticipation of his baby sister's moving into the nursery, we created a big-boy bedroom for my son. Some of the items moved with him from the nursery; others were thrifted; others were purchased new and one iconic feature, a reverse-stenciled dresser, was DIYed.
Also on these boards
love white/grey rooms
This site has great ideas for kids' parties and rooms. It will come in handy when it's time to redo the boys' rooms.