Take the ferry or sail to Spectacle Island, visit the museum on the island's history, hike up to the top of the island (the highest point in the harbor), enjoy the view, picnic.
Bike the Minuteman Trail from Alewife to Bedford, continue to Concord center, enjoy the old New England town, eat lunch and go antiquing/bookshopping, continue to Walden Pond, walk around, see Thoreau's house, swim in the pond.
Drive out to Sterling and navigate the Davis Mega Maze. Stop on the way back to go apple-picking and drink fresh cider.
Walk the Freedom Trail and stop in at some of the historical sights. Walk Newbury Street and all the touristy shops.
Rent a kayak in the Broad Canal (in Cambridge) and go up and down the Charles River. (http://www.paddleboston.com/kend...) Dock it at the kayak dock at the North Point Park, one of the area's most manicured and beautiful parks (built at a cost of tens of millions as part of the Big Dig to mollify environmentalists) that's also usually deserted and one of the most difficult to get to (both of the bridges originally planned to access the park were cut from the Big Dig after it ran over budget; one is now under construction again with federal stimulus money). Explore the park, kayak around the artificial islands, and try your luck at the incredibly dangerous but fun adult "goth" playground that tries to kill you and the spinny things on the intermediate playground. (http://www.yelp.com/biz/north-po...) Kayak back.