Leiden is a famous university city 30 minutes south of Amsterdam. It is a pretty strange city to explore and there is a “keys” trail you can follow, but it had me quite confused. Unlike Amsterdam, the city is not so packed with tourists, smart shops, and coffee houses. The city is mainly for the Dutch and several students.
There is a mega Cathedral in the center of the city, as well as, several towers and even a quirky Windmill Museum. Everything is easily walkable and since the streets are so small, its pretty much the only option. It is definitely worth seeing, if only to escape the Amsterdam crowds for the day.