Shipyard brewery is the big game in town, their old port brewery has tours but you don't see anything other than a video and do a tasting. I'm not sure Gritty's does a tour, but you should def. check out their bar in old port and/or freeport. Sebago brewing is another but I'm not sure they do tours either. Finally, Allagash is the place to check out a tour, although I haven't been on one myself.
Also...hit up Great Lost Bear for a great selection of Maine brews! Can't miss kind of place.
Have fun! This time of year is a great time to come out and visit us!