With only eight hours between flights you probably don't have time to leave the airport and tour Boston extensively, or go down to Fenway (not Fenwick
) park which is a bit of distance from the centre. What you could realistically do is go to Faneuil Hall / Quincy Market, which is a very pleasant area in summer, with an open air market, giant food hall, restaurants, street artists and other attractions.
Take a five minute bus ride from the airport terminal to the airport T (metro) station on the Blue Line. Travel two stops to Aquarium station. Alternatively, you can take a water shuttle from Logan to Long Wharf (the public MBTA shuttle is $3 each way, not sure how frequent it is, or there are more expensive private water taxis). The journey from Logan to Long Wharf is 12 minutes. By water or train you'll end up either end of the unmistakable Long Wharf Marriott hotel. From the Aquarium T stop it's less than a five minute walk to Quincy Market. Cross Atlantic Avenue, walk to your right and it will bring you to the bottom of the market. On the train either of the next two stops on the Blue line -- State Street or Government Centre -- will also give you easy access to Quincy Market. From the latter you can also walk to Boston Common (about ten minutes) or switch to the Red line and go a couple of stops to Park Street in about two minutes, which leaves you right on the edge of Boston Common. That, and the adjacent Boston Gardens are nice too.
The whole Faneuil Hall, State Street, Boston Common area is part of the
Boston Freedom Trail -- if you like walking this is a historic walking tour marked out by easy-to-follow red lines on the footpath. It starts near Park Street T stop, but you can join it anywhere you like (including Faneuil Hall at the top end of Quincy Market) -- get an online map and do some research before you go.
For my money this is your easiest and cheapest way of experiencing a bit of Boston while not straying too far from the airport. Obviously there's a lot more to Boston than this, but I've spent many a pleasant day just around this area. (I used to live in the middle of it).