Unfortunately, one of the idiotic realities of living in Boston is that the bars shut down at 2am and the T shuts down at 12:30. The T had a short-lived experiment in "Night Owl" service, where they ran buses until 3am on Friday/Saturday nights. Even though it was a very popular service, due to the outrageous overtime and salaries and all that they had to pay to run it, it was discontinued about 6 months ago.
So, your only option will be a taxi to Braintree, which will be about $25-30 with tip, unless one of you plays "designated driver".
For clubs/bars with dancing, your best bet is either Landsdowne St., just outside of Fenway Park, which is lined with clubs, Canal St. and the neighboring streets just off of Causeway St., across from the Banknorth Garden, or Boylston Place, which is a short alley lined with clubs, just off of Boylston St., near the intersection with Tremont. There are also a couple of places around the corner from there, in the Theater District.