Pak Nai: about 2 1/2 hrs drive from Nan city, a lake in the hills on the Nan/Uttaradit border with a car ferry and several floating raft restaurants. Basic accomodation available, too.
It makes a pleasant day out or a boozy overnight stay, karaoke, a variety of Thai dishes with heavy emphasis on fresh local fish and the usual Thai beers and liquors. You may do your own fishing, a bamboo rod is 10bt, great fun for the kids.
The last 10km across the hills are windy, and steep, about as much as one could comfortably do on a rented scooter. There's a car-ferry between the 2 provinces in operation.