the seediest bar in boston i had ever been to is on waltham st. in the south end, aptly named "the waltham"...i believe it's still open...what an adventure to go in there.
best places to look at young women are fanuel hall (mcfadden's, daisy buchanan's etc..) and i also like the Harvard ave./Comm. ave. intersection in brighton...there are maybe 20 fun young bars within two blocks.
Head over to Cambridge - Harvard Square is fun for those little eats places and people watching. There's a great beer bar over there and the name escapes me right now, maybe someone else can name it.
I love the Musuem of Science (mos.org) - just a fun place to go I think.
The Backbay --- Boylston St, Newbury St, etc is another good people place. There's the Hardrock on Clarendon St if you're collecting the tee-shirts. A stroll thru the Public Gardens or ice skating on Frog Pond.
Obviously if you like history do the Freedom Trail. But I like the Duck Tour too, it's fun. But the North End is great, although it's not a big feast time of the year, there's always something doing there.
If you like beer, the Sam Adams tour is good as is anything around Fanueil Hall/Gov't Center.
But you're here for one of the best holidays here - St. Patrick's Day! Head to Southie (South Boston, not the south end of Boston). The parade is fun and it's just one huge party there.
It's a fun place - enjoy yourself.
(Braintree doesn't have much a downtown hang out place so you really are out of luck there. )
There are several dozen pubs/eateries in Quincy Center - less than 10 minutes from most of Braintree.
If you prefer a sports bar there's The Fours. For Irish Pubs there's the Half Door and Bad Abott's. If you want Greek go over to X/Os. If you want something a little more upscale try Alba. In between those are several dozen Asian, Indian and Brazillian places. If the weather is decent take a trip down to Marina Bay - there are 8 or 9 places right there on the waterfront.