One day for Yamadera and Matsushima from Sendai is do-able but a bit rush. Half a day for Yamadera is OK and there is no need to stay overnight there. Matsushima deserves more than half a day. Besides the cruise, a leisurely walk in town is recommended to take in scenic Oshima, Godai-do, Kanran-tei, Zuigan-ji, Entsu-in and even further to Fukuura-jima. So try to allocate more time than just an afternoon for Matsushima. Ideally spend a night there and enjoy an oyster meal and hopefully catch the beautiful sunrise the next morning if your lodging faces the bay.
With 2 days and without changing hotels, you can do the 2 places as separate side trips from Sendai on different days. The rest of the 2 days can be used to tour Sendai itself. In Sendai you can take the convenient Loople sightseeing bus and hop-on-and-off between the city sites.
http://www.kotsu.city.sendai.jp/english/bus/loople/index.html 2 major sites are Zuihoden Mausoleum (stop no.4) and the castle park of Aoba-jo (stop no.6). Osaki Hachiman-jinja (stop no. 11) is also worth a look. Ichiban-cho (stop no. 2) on Aoba Dori is the covered shopping district and is beautiful lit up after dusk. It is within walking distance from Sendai JR station.
Not on the loople bus route but the following places are also worth considering if you have extra time in Sendai. Easily reached by 10 min subway is the Tomizawa Site Museum, an impressive well preserved fossilized forest dating back 20,000 years.
http://www.city.sendai.jp/kyouiku/chiteinomori/ About 30 min bus from JR station is Kanazaki Belle Factory where you can see those colourful paper floats used during the summer festival. Also on site are some hand-on experience including fish cake DIY.
http://www.kanezaki.co.jp/belle_factory/ Visiting in June, Rinno-ji can be a highlight as its garden is said to be blooming with beautiful iris. The Japanese garden has a carp-filled lotus pond with surrounding bamboo grove and a pagoda.
http://www.rinno-ji.or.jp/ If you are a cartoon fan or have children with you the latest attraction in Sendai is the Anpanman Children's Museum & Mall opened less than a year ago.
http://www.sendai-anpanman.jp/ En-route from Sendai to Matsushima is Mitsui Outlet Park which is said to be largest outlet mall in Tohoku. Besides shopping it also has a playland with a large ferris wheel.
http://www.31op.com/sendai/foreign/index_en.html Don't forget to try the famous gyutan in Sendai. You can get hold of a gyutan map of eating places in Sendai from the TIC office in the JR station. Even inside the JR station on the upper floor is a gyutan dori which has 4-5 small gyutan restaurants. The following map shows some restaurants around the station area. You can click the various locations for a detail description though the information is only in Japanese.
http://www.innerbrain.co.jp/mapsendaiekimae.htm Note that to be able to get the JR East 3 day flexipass special you need to arrive in Japan no later than Jun 21, 2012. Once you have the pass, you can use it till the end of June, provided you activate it from Jun 20 as its valid period is within 10 days.
http://www.jreast.co.jp/e/eastpass_sp/purchase.html Also note that if you decide to do Yamadera and Matsushima on separate days, you will need to pay extra for one of the trips as you will need to set aside 2 days to and from Tokyo. An alternative is to combine the shorter trip to Yamadera on the same day upon arriving at Sendai from Tokyo or on the day going back to Tokyo.
