Another good one is Amsterdam. Train station right in the airport takes you right to the city center in about 15 min. They actively promote it if you have a four or so hour layover or longer. First time I visited we had seven hours, we actually took the train into town, rented bikes for three hours for 10EUR-ish, and saw quite a bit of the sights (and by sights, I mostly mean the brewery in a windmill) that way.