I agree with the itinerary above. There is a small microbrewery just outside of Manchester,NH called White Birch Brewing. I checked and they're not having brewery tours that day but the store is open. I've never been there, it seems like a small operation, but I have had a bunch of their brews. Could be a good pit stop before hitting Portsmouth which is a lot more fun.
Heading south from Portsmouth there's Newburyport and the north shore. That's scenic with a lot of good bars. I'd take route 133 south. There's even a distillery out in Gloucester, Cape Anne Brewing and Salem Beerworks.
I live a couple of towns over from Lexington. They roll up the sidewalk by ten in the area. There are a bunch of good restaurants on the main street heading to the Lexington Battlefield. They have a good selection on tap but you're so close to Boston I'd head into town. Wouldn't take more than 30 minutes to be downtown.
I know I didn't add much to the conversation but I'd have to say make sure you stop by the Alchemist or at least find some Heady Topper. I'd also add Maine Beer Co.'s Lunch IPA to be on the lookout for.
** There is the British Company out in Westford. It's a chain restaurant but they have a good beer/cider list. That's not too far from Lexington and is still out in the sticks with nothing around it. They have Heady Topper on menu but never in stock. Head into Boston!
EDIT: Mass Brewer's Fest on the 24th in Boston in case your dates change