Use Chalkboard Paint: Katie of The Red Kitchen repurposed her old crib as a chalkboard easel! You can also use colored chalboard paint to make a height chart with chalboard markers. Get the DIY instructions here. Source: The Red Kitchen
If you want to keep your crib for sentimental reasons, but it takes up too much space why not turn it into a bench? --this would be so awesome to do with the twins cribs and use it at the kitchen table. Maybe alter them a bit but I like the idea!