Walking is the best way to see Boston. The narrow, twisting streets that make driving such a headache are a treat for pedestrians, who are never far from something worth seeing. The central city is compact -- walking quickly from one end to the other takes about an hour -- and abounds with historically and architecturally interesting buildings and neighborhoods.
Be sure to wear comfortable shoes, and if you're not inclined to pay designer prices for designer water, bring your own bottle and fill it at your hotel.