Museum of Fine Arts and Isabelle Stewart Gardner Museum if you're looking for some art.
Really just walking around the commons, Charles street,backbay Kenmore...
I'm with Redz. About the only thing I really like to do in Boston is walk the Commons / Public Gardens, Charles St., Back Bay and North End. Walk from there down to Newbury and Boylston Streets -- stores that real humans can't afford, but interesting. Hopefully you get nice weather.
The Boston Public Library on Boylston is an interesting take (tourists walk through it all day) -- just down the road from the Commons. Theater district is nearby too.
North end has lots of great Italian food, though I'm not in town enough in recent years to recommend anything specific. Near The Garden.