That depends on how you're going to get there.. are you going to be taking I-95 to Bangor, then down through Ellsworth, etc., to get there; or are you taking US Route 1 all the way up the coast?
There are some breweries right in Bar Harbor (check the list above).
Other good breweries: Allagash (Portland), Black Bear (brewpub in Orono), Gritty McDuff's (brewpub Portland, Freeport and Lewiston... I like the Freeport location best)... Sea Dog (brewpub in various locations.. the one in Bangor is nice.)
If you do make it to Bangor, I STRONGLY recommend you visit Nocturnum Drafthaus on 56 Main St. The owner, Gene, is not only a homebrewer, but he's featuring something like 14 beers on tap, including casks and lambics. The tap list from Saturday: http://www.nocturnemdrafthaus.com/2011/updated-6-4/
He gets his hands on a lot of stuff that no one else even bothers with. And let him know Jonny B sent ya. ;-)
Hope you enjoy your stay. :-)