When it comes to traveling to Boston from Maine or New Hampshire, one option stands head and shoulders above the others, in my humble opinion. That option is the highway coaches run by Concord Coach Lines.
Concord’s routes all end at either South Station or Logan Airport. The price is reasonable — $25.00 same-day round trip ticket through 3/31/10 — and the amenities are stellar. Some buses have power outlets in the seats, some have WiFi. Some offer water and a snack at the beginning of the trip. All have complimentary headsets for watching a movie or listening to one of four types of music. I have found these coaches always to be clean and comfortable, non-smoking, and equipped with bathroom.
Another point of note is that these coaches run on time! Portland, Maine, to walking around Boston in two hours. That is not going to happen with any other means of transport.
Boston, Mass, is not an easy city to navigate by car if one is not familiar with it. My suggestion? Make the most of your day by taking a Concord Trailways bus and walk it when you get there.