The Dutch national museum for the history of science and medicine, housed in a former convent that later served as a plague hospital. The collection is exceptional: early microscopes by Van Leeuwenhoek, pendulum clocks by Huygens, a reconstruction of the world's first cryogenics lab where Kamerlingh Onnes liquefied helium in 1908, and medical instruments that will make you grateful for modern anaesthesia.
The building itself is worth seeing — a handsome 16th-century complex arranged around a courtyard. The museum covers Dutch contributions to science from the Golden Age to the present, and since the Netherlands punches well above its weight in Nobel Prizes and scientific firsts, there is a lot of ground to cover. Named after Herman Boerhaave, the 18th-century Leiden physician who essentially invented clinical teaching. Allow at least two hours.