Dave makes a good suggestion about going due west to Portland. You can take Rte 302 to Rte 25 almost all the way, and that is a nice two lane ride. Here's a
Google Maps link of that route. The ride on 25 through to Plymouth is nice open sweepers and mountain scenic. You'd skirt the north end of Lake Winnipesaukee and then the ride from Ossipee, NH over to the Portland area is pretty nondescript. The ride will be about 4 hours, but you shouldn't hit too much traffic anywhere along the way.
I'd probably try to stay out of downtown Portland for lunch, maybe head just a bit further up the coast to Freeport or Yarmouth?
Harraseeket Lunch and Lobster Company is on the water in Freeport and is a good take. It is casual dining (including some picnic tables outdoors if you like). Ultra fresh seafood as the restaurant is right in the harbor where the lobster boats pull in. It is a quintessential Maine lobster pound experience, though a bit on the pricey side. There are a number of places along Rte 1 in Freeport, which is also the home of LL Bean, if you are into that kind of thing.
The
Royal River Grille House is a bit more formal, and nice, further south in Yarmouth. As the name infers it overlooks a marina on the Royal River inlet from Casco Bay. Nice views. Also pricey
Days Crab and Lobster is a more frugal take-out lobster shack with picnic tables, but it's right on Rte 1 in Yarmouth, (so not much ambiance there), Just fresh lobster rolls, crab cakes, and fried seafood.
If I had to choose one for you, I'd shoot for the Haraseeket L&L. We've gone there for dinner on the Chowder rides in the past, and I take my Mom there every once in a while (she lives in Portland).