I'm not sure about a theme that will be suitable through to teens years. But I've just decorated Nina's room in Disney Princess. She loves Princess stuff.
In her room is a bookshelf, her little princess lounge, bedside table with a princess lamp, her quilt cover is princess, and on the walls is a princess canvas with stars/heart stick on things beside the canvas. On another wall, I had her do a painting on a large canvas.
This didn't cost very much at all. The canvas painting she did cost me $15, the lamp cost $20, the quilt cover cost $50, the bookshelf we've had for a few years, and the bedside table cost $10.

And I will be decorating Emily's room in Fairies theme when she's a little older.