If you don't have time for the big musuems, the Rembrandt house is nice and fun.

At the risk of beating a dead horse, I will chime in: absolutely go to the Ann Frank house if you can. It's very moving -- one of the most memorable places I have ever visited in my four trips to Europe.

Then try to take a boat ride on the canal or just walk around the Leidesplein and soak up the beauty of the city.