the network for radical unschooling families
Off the beaten path I suggest a visit to The Garment District. It's a thrift/costume/retro store that is an adventure! Also if you like to dance, The Dance Complex in Cambridge has drop in dance classes, specifically African Dance with live drummers that draw all ages and experience levels. Cost is 10-12 dollars for drop in...a really fun and different thing to do. There are four levels of different kinds of dance happening.
Boston's Museum of Science is quite good and it has a very cool Omnimax theater. Definitely worth a visit. If you are at MIT, along the Charles River is a beautiful walk and the other side (Boston) is the Esplanade. Great for people-watching, strolling. I believe there are free family concerts every Wed. night at the Hatch shell which is where the 4th of July event is held. Community boating may have some options if you want to get out on the river. They are located near the Museum of Science on the Boston side of the river. Depending on your budget, whale watching from Long Wharf is an amazing experience, or even a trip out to the Harbor Islands. If you love history, there is plenty to see and do. The Freedom trail is free or you can use a guide, starting at the State House. The swan boats at the public garden are also quaint and fun and cheap. If he is into gaming or RPG, go to Pandemonium Books on Pleasant St. in Cambridge. The Miracle of Science near MIT is great for burgers and coke during the day. More of a bar crowd at night. Hope that helps. Have a great time.