Haarlem is very beautiful and well worth a visit. Might be a bit "boring" depending on what you like, but it's a charming and peaceful little town... the North Sea beach, Zandvoort racing circuit, Keukenhof Gardens (only open in spring though), Zaanse Schans windmills are all just a short trip away too.
Utrecht is also very nice, so is Leiden. Both essentially similar to Amsterdam, but less crowded and touristy. But I love the Netherlands in general. Regardless where you go, both bigger cities and small towns and even villages are so damn beautiful and have so much to offer.