Matsushima is worth visiting any time of the year. Enjoy a relaxing cruise on Matsushima Bay which is especially beautiful during sunset. End of the year is also oyster season in Matsushima region, so don't forget to have an oyster feast while visiting Matsushima.If you are looking for other side trips from Sendai, Yama-dera is a very good suggestion. The temple bldgs. are spectacularly perched on the mt. slope and the view on top is breath-taking. The only thing to note is that you have to climb about 1000 stone steps to reach the top.
The Mangattan Museum in Ishinomaki is also nice if you are a manga and anime fan. Apart from the museum, the whole town of Ishinomaki is full of street-side cartoon figures. The walk from the JR station to the museum will let you pass funny-looking cartoon sculptures and even a small manga shrine, lots of photo-taking opportunity!
If you want shopping, en-route from Sendai to Matsushima is Mitsui Outlet Park, the largest shopping outlet in the Tohoku region.
http://www.31op.com/sendai/
If you want an onsen experience near Sendai, consider Akiu Onsen which is easily reached by bus from downtown Sendai.
http://akiusato.jp/index.aspx
