Booked 6 nights in Tokyo. We’d like to take a train and stay at least one night somewhere else while we’re there. Where should we go?

Our hotel is 6 nights in Tokyo, but I’m regretting not trying to go multiple places. We definitely want to take a train and see more of the country, and we’re thinking if we see somewhere cool, we’ll just book a hotel last minute and stay the night instead of going all the way back to Tokyo. Is this a good plan? Any suggestions?