Leiden is a middle-sized city in Holland, with around 100,000 inhabitants. Leiden is only 20 km for The Hague, and not much further away from Rotterdam, so if you are in for some action, you can combine all three cities on a day trip with train from Amsterdam (Leiden is about 40 km from Amsterdam).
Leiden has been a university city since the 16th century, and it is not a coincidence that Leiden is a sister city to Oxford. The city might not be full of amazing buildings, but it is a very typical Dutch city, with a nice city center, a great atmosphere and long walking streets.
The two most impressive churches in Leiden are the Pieterskerk (construction started in 1390) and the Hooglandse Kerk (construction started in 1377). Besides these there are many small museums and interesting buildings, and visitors are recommend to visit “Pannekoekenhuysje Oudt Leyden”, the oldest pancake house in the world.