i joined a lot of FB Groups. I enjoy hiking and running and I groups like that specifically for Southern NH. They have a lot of meet ups. Also going to breweries they have events that are fun even if you dont drink. I know 603 brewery puts on events.

Portsmouth sounds like it would fit your criteria too! A really beautiful town. Very walkable, lots of parks and trails. The downtown has bars and a bit of a nightlife. By the water/beahces (right by york, ME) but also close to the mountains. Lot of young people too and everyone is soo friendly. The food is amazing too. Trader joes and whole foods close by.