You can rent a hubway bike and bike along the Charles seeing Harvard, BU, and MIT. Fun stuff. https://www.thehubway.com/ also download the UBER app so you can use it when it is very late-when you might be hesitant to ride the T (although there is no particular reason not to). Have breakfast at Friendly Toast in Kendall Square.
What about a visit to Fenway Park??? I second the recommendation about the Isabella Stewardt Gardner museum. The MFA is great. It has been a very long time since I’ve been there but the John F Kennedy Library is really interesting.
@palm715, you are not frumpy. You are one of the most linguistically svelte posters on this site.