Glad you will be visiting! I’ll chime in with a few lunches we’ve enjoyed in Portsmouth and Portland so far this year.
In Portsmouth (NH) we have made a lunch stop at Row 34 numerous times over the years on our way up to Maine. Their oyster sourcing is impeccable, and their lobster rolls are among the best you’ll find anywhere. (That said, this year the prices of lobster rolls most everywhere blow my mind. So I will offer a less stratospherically priced, top-notch option in Portland.) Row 34 is also deservedly known for its curated selection of beers that you won’t find everywhere.
Photos from an oyster, fish sandwich, and fish taco lunch that we had at Row 34 back in March.
In Portland (ME) twice this year we have enjoyed a stop at Bite Into Maine’s Diamond Street location. Lobster rolls here are top-notch, with tender, fresh lobster (rubbery lobster is, sadly, a thing). Note that this location is not at all picturesque, but it is within a short walk of great coffee (Coffee by Design) and a trio of taprooms (Rising Tide, Austin Street, and the gluten-free Orange Bike).
A recent July lunch. Melted butter is an option here, which I think everybody should experience at least once:
Another stop in Portland that we enjoyed tremendously was Terlingua. Wanted to go for years, and back in March finally had the chance. The two of us shared this app of deviled eggs and a barbecue platter. We felt very well-fed.
Note: Getting a parking spot on Forest Ave. can be tricky, so our server suggested trying the streets behind Terlingua next time. Haven’t tried this tip yet though.
No pic, but if you find yourselves strolling in Portland a stop at Gorgeous Gelato is a treat. (I’m an admitted gelato snob so not a fan of the better known Gelato Fiasco, which just doesn’t do it for me.)
Only offering recent personal experiences but there’s so much more! For Portland (ME), you can find an extensive list of restaurants and info at Portland Food Map.
P.S. Echoing what @digga notes above regarding traffic. We try to avoid peak travel hours to maintain happiness. Lots of fellow vacationers on the road during these peak summer weeks. Especially on weekends.