Lots of great suggestions, I would add hitting the Seaport. My BC senior has spent a lot of time there during her time in Boston. It has great restaurants and shopping. Very easy to get to via public transportation as it is a quick walk from south station, a main hub.
If you go to the North end, skip Mike’s. Head to Modern Pastry instead, or better yet Bova’s. Although good, Mike’s is known as a tourist trap among locals.