We chose Valkenburg, Netherlands which is just a 15 minute drive from our house.
A bit of history for those that are interested...(for those that aren't, skip the next paragraph)
Famous for its ancient limestone caves, Valkenburg is where the Netherlands can trace its history back for centuries on end. A fascinating assortment of charcoal drawings, bas-reliefs and other forms of artistic expression in the caves offer an inspiring glimpse of life here once upon a time. From Roman times, when the occupying forces knew of the caves to World War II, when countless refugees sought shelter here. For those interested in age-old mysteries, there are secret passageways connecting the Fluwelengrot, or velvet cave, to the castle. In fact, a 70-kilometer network of underground passages filled with museums and other attractions runs through and around Valkenburg. (For even more information... http://en.wikipedia.org/wiki/Valkenburg_aan_de_Geu l)