Starting with Sendai, the biggest attraction is Matsushima – one of Japan’s 3 scenic views. How will you be traveling? By train or by taxi/car? It seems that Matsushima Kaigan Station only has stairs! So, you may need a taxi… However, a popular option is to go to Shiohama station and take a ferry through the islands. The company has at least three accessible (“barrier free”) boats. You can contact them via their website: https://www.marubun-kisen.com/english/matsushima/list.html
After that there are a few temples. It may be easier for you to ask about certain places and I can research them for you.
Hakodate has a number of places to see that are worth it.
Mount Hakodate is very famous for its night view due to the unique geography of the area. The rope-way and facilities are accessible.
Fort Goryokaku is accessible (with the exception of one or two buildings) as is the observation tour next to it
The Morning Market is delicious, but many of the stores are not accessible (but some are)
The Red Brick Warehouses are mostly accessible
Motomachi – the residences are not accessible, but you can still view them from the grounds. Since they are very Western, they may not be of interest to you anyway!
The biggest problem in Hakodate is getting around. There is a trolly, but most of the stations are not accessible, and even those that are “accessible” require a very sharp turn onto a steeps ramp. Additionally, Motomachi and the Mount Hakodate rope-way base station require going up a very steep hill to even get to them. You may want to consider renting a wheelchair taxi.
I’m not really sure about Oarai other than that it has an accessible beach: