Martin in Fonthill, I don't know where your crossing the border but If you like country roads I'd get to RT 2 (the Mohawk Trail) at the Vt, N.Y., Mass border and follow it east to Rt 495N and at the Mass, N.H. border switch to Rt 1 and head north. That time of year will be very crowded on Rt 1 but it kind of follows the coastline with a switching off to Rt 1A from time to time. If your a person who camps at Wally World there should be plenty of them on that route.
If you'd rather go the Mountain route then cross over in the Lake Champlain area in North East N.Y. That would be Canada Rt 15 which turns to Rt 87 in N.Y. then swing east in the Alburg area, there is a nice Vt. state park in North Hero Vt. Pick up Rt 15 to Rt 93 in St Johnsbury go south on 93 and you'll pas where the Old Man In the Mountain used to be and the Flume is right in that area, just north of Lincoln, NH you'll want to go east on Rt 112 which is the Kancamagus hwy where you will want to take your time as there are a dozen or so camping areas in that area and it is a gorgeous 37 mile ride. once you get on the eastern side of NH you'll end up in Conway, NH then take Rt 302 east and from that point I'll leave you on your own.
The mountain way would be my choice, you won't be dissappointed.
Have fun, Bill