More on Insurance as well as a Few Stops
In addition to checking with you own personal car insurer, it would be worth your time and effort to check with your credit card to see if it offers any automatic coverage if you charge the car hire to it (ours does), and with any auto club (RAC, AA, etc,) you may belong to. Coverage may be automatic (and free!) or you may need to sign up for it, but either way it will be cheaper than what the car hire company is going to charge you.
As for some specific suggestions for things to see between Bar Harbor and Montréal, consider first heading almost directly west into central New Hampshire, say via US-2 to Gorham. This would put you in a position to visit some combination of Mount Washington, Crawford Notch State Park, and/or the Kancamagus Highway in New Hampshire, and then in Vermont: Ben and Jerry's, the Trapp Family Lodge, Hope Cemetery (yes, a cemetery as a tourist attraction!), and/or Church Street Market in Burlington. The more direct route from Bar Harbor to Montréal (ME-16/ME-27 to QC-212 through Sherbrooke) goes through the Maine north woods and the Eastern Townships of Québec and while pretty, there's not a lot to see and do along those roads.
Then from Montréal down to New York City, as noted before, you'll be following an historic and well-traveled route that hugs Lake Champlain and the Hudson River. Some sites of interest: Adirondack State Park, Ausible Chasm, Crown Point, Fort Ticonderoga, Catskill State Park, Hyde Park, West Point, and Harriman State Park.
AZBuck
Slightly Longer, But Worth It
Strictly on a cost basis, the Charlotte (VT) - Essex (NY) crossing is the better deal. The car and three passengers will run you about $23. The Burlington - Port Kent ferry, by contrast, would run you about $54. You will have to drive a bit farther to use the more southern (cheaper) ferry, but this is a RoadTrip, No? My wife and I have used the Charlotte-Essex ferry, but not with a car. We used it as a relaxing scenic 'cruise', walked around Essex for an hour or so and then caught the ferry back to our car which we had left on the Vermont side. Very relaxing.
AZBuck
Keep It As Simple, Straightforward, and Self-Reliant as Possible
You're now looking to do one of the most nerve wracking drives in America on the fly, with time constraints, and with a balky GPS (Satnav). This is why planning, and self-reliance are so important to a successful RoadTrip. Your best bet at this point is to:
1) Get good paper maps of both New York State and New York City. These should be available from almost any convenience store or gas station for a few dollars.
2) Familiarize yourselves, both of you but especially the navigator, with the route I'm going to recommend. Mark it on the map if you have ANY doubts.
3) Turn off 'Bruce'. If he's got a time lag, he WILL lead you down the wrong road at some point. You do NOT want erroneous or conflicting advice on this drive. You will be on your own, but that's better than getting bad directions and believing them.
4) Leave West Point around 9:00 AM. That should be after the morning rush hour and still leave you plenty of time to get to JFK by 11:30. You do NOT want to cut this any closer than that. If you are planning to drop off the car at 11:30, then I hope your international flight is no earlier than 3:00 or so in the afternoon. If it is earlier than that, leave West Point that much earlier.
Now as to route. What I'm going to recommend is not the shortest route or the route that any GPS would recommend, but you want to make as few turns/connections as possible and you want to avoid the Cross Bronx Expressway if at all possible. So...
Leave West Point and take US-9W south to the Bear Mountain Bridge, cross it over to the east side of the Hudson, and pick up US-202 and take it south (NB: this may be marked as US-202 north!) to Peekskill where you will leave US-202/US-6 and take US-9 south. A couple of miles south of Croton-On-Hudson, watch for NY-9A and make a left onto it. Follow that to the Taconic State Parkway south. Stay on that even as the name changes to the Sprain Brook Parkway. Take Exit 11E to the Cross County Parkway east to Exit 9 and the Hutchinson River Parkway south. Get in the left lane and stay there. This will eventually become I-678 and take you the rest of the way to JFK. Follow the signs for Car Rental Return.
Again, and I can't stress this enough.... Turn Bruce off. Get good paper maps of both the state and the city. Mark the above route and familiarize yourself with it. As you drive, it is critical that the navigator maintain positional awareness and know at all times how far it is to the next turn or change of route and exactly what you're going to do there. And make sure you leave yourselves plenty of time.
AZBuck