Portsmouth is a great walking city as many have recommended. They have outdoor ice skating in the historic strawberry bank settlement, which is absolutely magical, and lots of good restaurants, like Moxie and Street, and Liars Bench is a cool brewery

As far as restaurants and eating you'll be pretty disappointed compared to Europe for the most part. If you might want to explore Manchester (biased cause it's my home), it's probably an easier drive from gunstock than Portsmouth. People tend to shit on Manchester, but it has some of the best restaurants in the state. Elm st and Hanover st have several great places and I'd recommend Industry East (doesn't accept reservations, but has the best food in town), Firefly, Ignite (Mahi mahi dish), the Crown, and the Foundry. Birch on Elm is also reopening soon. I would definitely recommend driving past Concord and heading to Manchester. Concord doesn't have as good a food scene imho.

Up near gunstock all I can really recommend first hand is the brewery Kettlehead.

Edit: I could make an argument for spending a night in Manchester rather than Portsmouth. The drive up 93 from is substantially easier imho.