A tip for people looking for a decent coffee shop.. if you don't already know about it, you will because most Contiki travellers find out about it - its The Grasshopper. Ask a local and they should be able to tell you where to find it.
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.
Definitely worth doing if you are in any way interested in Ancient Rome - the guide is a wealth of knowledge.