I was just there. You will see Cape Cod beer co everywhere. Beer is okay, but their company is pretty cool. I think they do tours on Saturdays, but you can stop in for a tasting anytime.
If you get to Plymouth check out mayflower. We didn't make it to the brewery, but all the beer I had was excellent. There is Off Shore on Martha's Vineyard, which is a brewpub. Nantucket brewing on Nantucket.
You can also find a ton of Mass breweries in the package stores. Every one we visited was pretty well stocked.
Liam Maguire's (sp?) was a good pub with a nice selection, as well. It was in Falmouth, I think. There is also a chain of pubs called Brittish Beer Company that all have good selections, as well.