Take a walk along the beaches in South Boston and visit Castle Island. You can also take a boat trip to the islands in Boston Harbor.
Harvard has lots of interesting museums to explore, as does MIT.
Walk or bike along the Charles River, or rent a kayak!
Go visit the Arnold Arboretum.
Take the commuter rail to Singing Beach, Newburyport or Rockport.
Red Sox at Fenway is always entertaining.
Check out South End restraints--yum!

Excursion to Marblehead and Revere Beach

Red Sox game or a Fenway tour.

Go see the new MLK statue.

Ferry to Salem. See the Hawthorne family cemetery. Take some pictures in Halloween costumes. Explore the sites. I thought it was so much fun. I think I was there for Halloween, though. Really fun.

The ICA and the Isabella Stewart Gardener museum are both fun, but not really off the beaten path. Listen to music at club passim?

Thanks for the ideas, these are great!

As to why we're going again, family visits, plus our whole family enjoys the city.

You can go on a chocolate factory tour in Somerville - we went many years ago and it was fun: https://www.tazachocolate.com/pages/take-a-tour

Peabody Essex museum? Decordova sculpture park? Eating tour of north end or Allston?

If you've been there so many times, why are you going again?

Heading to Boston soon with 2 teens. We've been there many times before, so have a pretty good idea of things to do, but I'm looking for ideas that might be a little more niche or off the beaten path. Kids are into museums, ok with historical sites (probably wouldn't want to spend tons of time there, but into seeing things), and just exploring random stuff. We will have a car and also T passes.