The trolley tour of Boston, offered as an optional, is the best way to see the entire city. Take it at your own pace, but I would suggest stopping off at the USS Constitution and Fenway.
I travelled on the European Contrasts tour in May 2006 with my boyfriend. We had 5 couples on our tour. We all had a great time and did not have any hassles being that we were couples.
Do not travel with ONLY a credit card and/or debit card. Why? Cards can become demagnetized or lost….you should have some cash or traveler's checks to fall back on in case of an unfortunate inconvenience involving your plastic.