Sendai is also just another 30 min away by train from Matsushima, which has one of the "three best" views in Japan. I did the whole thing as a daytrip from Tokyo (using the JR pass). The boat trip in the bay is pretty enjoyable--at least in the low season, where the boat is very empty. I imagine I would have a very different perspective if the boat was say even 60% full! Winter is also oyster season, and the oysters in Matsushima were great. Otherwise, I didn't have too much extra time to explore the town.

I did think about booking either the HI in Sendai or Mito, but they were either sold out for the weekend or very expensive.