Ueda is a small Nagano city with a sharper identity than its size suggests. It was the castle town of the Sanada clan, whose stubborn defence against Tokugawa forces made the ruined castle more famous than its surviving walls might suggest.
The town centre is easy: Ueda Castle Park, the city museum, old streets, and a few cafes can fill a relaxed half day. The stronger reason to stay longer is west of town at Bessho Onsen, where temples sit against the hills and Anrakuji holds Japan's only octagonal three-storey pagoda.
Ueda is not as dramatic as Matsumoto or as famous as Nagano, but it is a good spare day if you are already moving through eastern Nagano. Come for Sanada history, temple architecture, and a slower onsen-town mood.