Sounds like a pretty awesome tour! Have you attended Japanese baseball games before? I'm guessing yes.
I don't think Sendai is so much a siteseeing place (in town) as it is a place to check out local traditional handcrafts.
this page has a decent introduction to them. What to see would depend on what crafts interest you. The "side trips" listed
here would all be reasonable half day choices as well, I think.
Sapporo to Sendai is indeed a long train ride. Have you considered flying from Sapporo to Sendai?