Haven't been to Sendai, but Takayama is lovely, and so is Kanazawa.

For extra relief from the summer heat, take the following route to Tokyo--

1. Bus to Shinshimshima over the mountains (change at Hirayu after you leave Takayama). The route is gorgeous, with mountain views, takes you over timberline, and features from slightly intimidating curves and switchbacks.

2. Train into Matsumoto

3. Pick up the train to Shiojiri at Matsumoto (short hop). At Shiojiri, pick up the Chuo-sen to Tokyo/Shinjuku. More scenery as you travel along the shores of Lake Suwa and go behind Mount Fuji (which probably won't be visible). You'll hit the outskirts of Tokyo about an hour before you arrive at Shinjuku.