Boston is a fun, clean and fairly safe large city. They have put a lot of money and energy into turning the city around. The bay is awesome and there are so many little islands to visit, you can forget you're visiting a large city. It's also near Salem, which is a must see.
I recommend doing a tour of the Boston common! And you HAVE to get a lobster roll!
The cobbled streets. Great public transport links compared to other US cities
Plenty of history, great food market, good bars and lots to do. Very enjoyable. Found the locals to go the extra mile to help.