Hi Grant,
One option might be to hire a livery/car service (black car sedan) and have them take you to the rental car location where you can get to one way driving or even just to the airport to get home from that airport
IT would probably be around $100 or so including tip
THere are AMtrack locations in Concord, North Conway, and the like.
www.amtrak.com
have the car service take you to the train, and it would run from there down to boston and them from boston out