For day trips out of Boston you can use the MBTA commuter rail system (www.mbta.com). You could head north towards Gloucester/Rockport and the ocean, south to the city of Providence in Rhode Island, or west towards my neck of the woods and check out the history (Paul Revere, the Minutemen vs. the British, etc.) in Concord. If you're looking for a longer trip, you can take Amtrak from Boston up to Portland, Maine, which is an interesting coastal city. If you need more info, feel free to PM me.