I agree with everyone else: take the train or subway in. You will save lots of hassle, and probably not be much different in terms of money given the prices for parking in Boston proper (if you can find it). Navigating Boston roads by car is challenging, and managing among the Boston drivers is a separate but I think equally daunting challenge. Getting around on foot and by subway is much handier in my opinion.
Driving under Boston isn't too bad, at least during non-rush-hour times, but that's not at all helpful if you want to see Boston.
Besides the Freedom Trail (and Old Ironsides), you might want to see the New England Aquarium (nicely done, with an enormous central tank), the Museum of Science (and especially the Van de Graf generator show), the excellent Museum of Fine Arts, and the Gardner museum...and probably a bunch of other destinations.