Food.
Watch City Brewing Company
http://watchcitybrew.com/
If you're lucky, they'll have a batch of bacon beer. It usually sells out in a day.
The trail is a good walk through town. It's cool because you are in amongst the regular stuff of the city but you have a path and a purpose through it all.
The aquarium down at tha habah (that's "the harbor" for all you furrenners) is cool.
In fact the whole waterfront area is pretty cool to just hang out.
Faneuil Hall is fun with shops and lots of restaurants, buskers and such.
North end is little Italy and I think Saturday nights usually turn into street parties with the locals setting up shop on the street. Lots of food. I was walking back to downtown after doing the trail all day and a street party just started to build around me as I went. It was pretty cool. So I stopped and had dinner.
The south shores of Maine are only an hour away. Lots of beach opportunity if that's your thing. Ogunquit is probably the nicest but that town caters to a certain "orientation". Not that there's anything wrong with that, but if you have little ones in tow, be prepared to do some explaining.
Kittery, Maine is closer than Ogunquit and the attraction there is shopping in the outlet malls.
Have fun!
