The freedom trail, fanueil hall, quincy market, the old north church (think paul revere) and the swan boats are all the touristy things to do... and all kind of fun though on a holiday weekend they will be packed
There's a bunch of bars around Fenway that are fun to hang out and watch the drunks play... plus give yourself some time to wander around Fenway... and the green monster - its definitely an old school park and as much as I hate the Red Sox they aren't too many parks left like it anymore..
Newbury street has all the fancy schmancy shops for window shopping and a nice outdoor cafe or two for lunch.
Cambridge/Harvard/MIT are nice for wandering around...
Ghe timing m ight be off - but the Coit Observatory at Boston University is kinda cool - though I Know they arent open year round..
The Bull and Finch Pub on Beacon Street is the inspiration behind Cheers... pop in for a drink
Lansdowne street has a lot of nightlife and probably a lot of drunken college co-eds
and it's almost across the street from fenway - though the bars on yawkey at game time are more fun
a note on the freedom trail -wear walking shoes - girlie sandals won't cut it - most of the trail is cobblestone or brick and girlie shoes will end up in broken ankles..
At the NavyYard is the USS COnstitution (old Ironsides) and is pretty impressive to see..
the new england aquarium is pretty cool if you like that sort of thing
there are a bunch of whale watching type boat tours that you can take which you might or might not see whales but the boat ride is always nice.. .
the museum of science is pretty cool - better for kids though and the Museum of FIne Arts is alsovery nice - but really better on a rainy day