Aquarium and Science Museum are standbys. The Arnold Arboretum is nice for a long walk. A walk along the Charles River.

If you like colleges, you could walk around Harvard, MIT, BU, Northeastern, BC.

A walk though Boston neighborhoods might be nice. North End, Charlestown, Back Bay, etc.

Something more challenging would be a hike in NH if you are equipped and experienced to do so at this time of year. I do not know of the state of the ski slopes right now - we've had a lot of snow in the past two weeks but it has also been unusually warm.