The wonderful Matsumoto Castle should be first on everyone’s list, but the city does have plenty of other charms. The Nakamachi district is a maze of attractive Edo-period streets, home to a huge selection of cafes and galleries, and you could easily lose a few hours just strolling around. There are alpine views all around, so you might just find yourself a nice spot for lunch and end up staying far too long!
If you’re feeling a little more active (and it isn’t the middle of winter….), head to the Tourist Office for information about some great day-hikes in the area so you can explore the alps a little more closely.