Daizahoshi Pond is a lovely place to get away from the bustle of daily life. See if any water activities are available here, or simply soak up the peaceful ambiance.
You'll have no end of sightseeing choices in Nagano. Daizahoshi Pond is a good place to begin here, but there are numerous other worthwhile places to check out. Nagano Prefectural Shinano Art Museum, Zenko-ji Temple, and Togakushi Shrine are close to the body of water and will fill in your day nicely.