I'm not sure what your definition of fun is, but taking a walk on Oshima is a great way to spend an hour or two while visiting Matsushima.
Oshima is free, and is about 5-10 minutes away from Matsushima-Kaigan Station. It's a little tricky to find but there are some signs and if you try to stick to the roads closest to the shore you should find it soon enough. From the station, walk toward the water and MarinePla aquarium, then follow the small road to the right around the aquarium. TA little ways down you'll come to a wooden building (there's a large sign for the park management office at the street); turn left immediately after that (there's a large map of Oshima across the path from the building). You'll go down a street that turns into a path that leads right to the bridge.
There are three islands reached by footbridge from Matsushima: Oshima, the small island that is the home of Godaido, and Fukuura Island. This is the southernmost island. There are a few islands around Matsushima itself, but the real collection is further out, around Miyato Island. The cruise ships will come out here, but I think visiting with your own boat and exploring the random islands would be ideal. I've read that there are lots of beautiful caves to be found that the tour boats don't visit.