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.

ankopsu Article on Boston

I went on two tours alone...I made life-long friends along the way! That's the nature of Contiki.

efti Article on Travelling alone

This is awesome - you have to do it - so much information about the city - it really has an amazing history.

Mike Contiki Article on Berlin Historical Walking Tour, Berlin