I went alone last summer (06). I met some wonderful people…I liked it so much, that I am going on the London + Paris trip next week (alone again).
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 went on two tours alone...I made life-long friends along the way! That's the nature of Contiki.