As previously mentioned, start here:
http://www.scancapecod.us/wiki/index.php/Eastern_MA_Portal
Don't let the fact that the site's named "Scan Cape Cod" -- it's got some of the best, most up to date, info on Boston (and all of MA) anywhere. And it's well maintained.
In Boston, you'll enjoy listening to the Boston Police. See the BPD page linked to from the page above. You'll want to program their Channels 01 through 07 and 12 into your scanner (all conventional, no P25, no trunking for BPD).
For Boston Fire, see the page linked from above. If you just want to hear dispatch operations, channel 1 is all you'll need. For completeness, add a fireground or two.
This extremely useful site has lots of good info:
http://home.comcast.net/~garys2/scanguide/boston.htm
If you care about EMS dispatch stuff, check out:
http://www.bppa.org/bppaems/BOSTEMS.htm
After that, things tend to get a bit less interesting: The BAPERN channels can sometimes be amusing... they're also linked from the ScanCapeCod page I provided above.
If you're up for programming a large, complex, multi-site, trunked system program the Mass State Police Zone 5:
http://users.adelphia.net/~kc1ho/ematrunk/msp/msp.html
Beware there are a ton of talkgroups, and you'll need to sit through tons of boring license checks and general highway related stuff before you hear a good chase or drug operation.
Enjoy your trip!
de Peter K1PGV