Do you have Rick Steves Amsterdam and the Netherlands? He describes a wide variety of day trips, from cities to small towns to rural areas, in far greater detail than anyone can in a forum post. He also includes goof-proof directions on how to see each place as a day trip from Amsterdam, and includes accommodations for some of them if you want to make it an overnight (particularly the longer daytrips like the Kröller-Müller Museum).
Personally, I liked Utrecht, Leiden, and Rotterdam, and was less taken with the Hague and Delft. I was interested in cities, though, so none of these places will appeal to you if you want more rural destinations.