You don't need to drive in Boston, and you're better off if you don't!
Take the airport's free shuttle to the subway (the "T"), and take the Blue line to State Street. The fare is 85 cents and the trip takes about 15 minutes, depending on how fast the shuttle bus comes. From there you're less than a block from Fanueil Hall/Quincy Market, and on the Freedom Trail. If you're at all interested in the Colonial and Revolutionary War periods, I strongly recommend walking at least part of the Freedom Trail, which leads to many of the famous places of the period as well as some places that really have no historical importance but are just nice to look at. The Trail is something like 2 1/2 miles long, but most of the sights at the beginning are grouped closely together, and only the USS Constitution and Bunker Hill are too far away for your limited time. Also, Quincy Market has a range of good places to eat, as well as the usual horrible fast food. One of my favorites, which is actually a couple of minutes walk away, is the Old Union Oyster House. The food is merely good, but the restaurant itself, particularly the street-level dining room, is wonderfully atmospheric. I'm pretty sure it's the oldest continuously operating restaurant in the US.
I used to live in Boston, and I still love it, so if you want more ideas, please feel free to email me.
Enjoy.
(edited for egregious typos)
[This message has been edited by Kitty Hawk (edited 10-19-2000).]