Alkmaar Kaasmarkt (Cheese Market) - Yes, it's true. Hoorn, Gouda, Woerden and Edam have them, too, but if you'd like to see Holland's most renowned cheese market, you have to go to Alkmaar. Every Friday huge wheels of cheese are carried around and weighed on the old town market, which is quite the sight. The market is held every Friday 10:00 - 12:30 from April to the Beginning of September. Trains depart from Amsterdam for Alkmaar four times every hour.
Delft - Yes, there's also an Oude Kerk (Old Church), but the Nieuwe Kerk is where all the deceased members of the Dutch royal family are buried. Even legendary William of Orange rests here for all eternity. It's quite impressive and a must-see if you're in Delft as is the Royal Delft factory tour. See how Holland's most famous blue porcelain has been made for centuries (www.royaldelft.com).
Frans Hals Museum (Haarlem) - Sick of the same museums around Museumplein in Amsterdam? The Frans Hals Museum in Haarlem displays an amazing collection of paintings by Dutch old masters and is the museum to visit if you appreciate Dutch Golden Age art. Haarlem is close to Amsterdam and trains run frequently between the two cities.
Keukenhof - You can't really escape it. Flowers are a part of Dutch culture. Tulips, which were actually originally imported (smuggled) from Turkey, are a national symbol of Holland and the Keukenhof is the place to see them 'in the wild' once the season starts in early spring. The huge gardens are only open to the public for two months each year so visit www.keukenhof.nl for more information. You can usually arrange a trip at your hotel reception.