I made a giant ruler to hang on the door to track the kids' height at the beginning of each school year -- I copied this idea:
http://doorcouture.com/collections : scroll through until you get the pic of the door with the ruler, click on it, then you can register and get pdf instructions -- you can just figure it out from the photo if you don't want to register, but they've never sent me email or spam. I used real wood (cut to size for free at Lowe's) instead of foam board so it should last forever.
We take photos of it on the first day of school each year, and it makes a cute door decoration through September.