1. Taking in a Red Sox game at Fenway should be on every man's bucket list.
2. If you're going to be there on the 4th. Boston has an awesome Independence Day Celebration on the Charles River with the Boston Pops. I'm not usually in to that kind of thing, but it's freaking epic.
3. If you like historical reenactment theme parky stuff, Old Sturbridge Villiage used to be pretty awesome. It's about an hour west of Boston though.
4. Boston area has tons of interesting restaurants from the touristy places like Durgin Park to great food on the North End. There's a robust business in food/beer tasting tours in town that you might look into.
5. Boston has very good museums if you're into that kind of thing.
6. The Freedom Trail is more touristy than historically interesting IMHO, but you'll see a lot of stuff on the way. I'd check the route out online and see if it floats your boat.
7. The Springfield/Agawam area (even further west, but MA is a dinky state if you have a car) has the Basketball Hall of Fame and the Springfield Armory (weapons museum). Smith and Wesson is there too if they still offer tours under MA Jackbooted climate.
8. Providencetown (Cape) is a really pretty place since you said you might head to the cape. Check ahead because drag queens tend to take over some times during the summer and head somewhere else like Nantucket, Falmouth, Hyannis, or Martha's Vineyard.
Been a while since I lived there, so I'll have to think of other suggestions I'd have.
