If you are worried about pick pocketers go to www.pacsafe.com they have slash proof bags and purses as well as camera straps. Great for guys and gals. I used the citysafe 200 and its great.
Take 50€ and put it somewhere safe, your suitcase, your travel documents, wherever. This way you'll have it just in case.
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.