7 days in Tohoku Region

My partner and I are traveling Japan for 22 days!

The first 14 days are with his sister, which includes all the major places in and around KYOTO, OSAKA and TOKYO.

Once she leaves we only have 7 days left to explore the rest, and we were thinking of going up north but are lost at the idea.... it seems like 7 days is not enough time to go north as travel time is more time consuming, can anyone recommend any key places that we MUST check out near and around SENDAI? suggestions would be great!

Sendai - nice town, no tourists!! Go to Matsushima and get the ferry sightseeing trip (and the supplement to go upstairs (Y1500 plus Y600 for the outside viewing area). Very interesting place to explore once back on land too.

As for other areas further north, we'll be exploring those in July. My snowboarding friends recommend the spas/onsen around Zao.

There's SO much to see in Tohoku! :)
If you want to stay near Sendai, how about visiting Tashirojima, also known as Cat Island?:
http://zoomingjapan.com/travel/tashirojima-cat-island/

Other great sights in Tohoku are:
http://zoomingjapan.com/tag/tohoku/

If you stay along the Shinkansen, travel times are very short (it takes a little over three hours from Tokyo to Aomori).

Unless you want to stay at fancy places, I recommend you stay in cheap business hotels, say 4 nights in Sendai and 3 in Aomori (or the other way around), then make day trips (or museums, shopping, etc. if the weather is not nice).

From Sendai I'd recommend (from the top of my head, in no particular order) : Goshikinuma and the Mount Bandai area, Lake Tazawa and Kakunodate (also Nyuto Onsen if that's your thing), Yamadera and Hiraizumi/Geibikei. (No, I don't consider Matsushima a must-see, but feel free to ignore my opinion and go it you want.)

From Aomori, Osorezan, lake Towada and Hirosaki. If you feel adventurous, the Shirakami coast (Juniko especially).

Which date(season/month) starts???

Good or not is depends on timing.