I've got to agree with some above, and disagree with others.
If you're on a bike or are traffic averse, avoid the core leaf season like the plague, and along with it roads like VT100 and the Kancamagus (eastern 112 in NH).
During peak leaf season, accomodations are very scarce and very expensive in north / central NH and just about the whole of east / central VT. Traffic is at a peak, and moves even slower than conditions would suggest as everybody is watching the scenery, taking photos, disciplining the kids, whatever.
By far my favorite time of the year to ride is between Labor Day and the 3rd week in September. Tourists, mosquitos and other pests have pretty much disappeared, the weather is great (especially on the coast), and it's mostly too early to snow.
VT100: IMHO to be avoided on weekends, very pretty, but advertised in just about every tourist book and journal covering NNE that you will find.
NH112 / Kancamagus: west of Lincoln is some very good riding. East of that is RV heaven, unless you are riding on a weekday and out of season.
I cannot believe that a lister above warned against the Lincoln Gap and 108. I've done all the gaps and notches in NH on my ZX12R, VFR800, and this is very doable on a street bike if you're at all competent. I've got a list of all the notches and gaps in NH, VT and ME over on
http://groups.msn.com/NHConcoursOwnersGroup/thenotches.msnw. Some other stuff about NH roads over there if you click around about. It's a little out of date, but still some useful info.
Your best bet is to get the DeLorme atlas of VT and NH. For a freebie, the official VT tourist map is pretty good as well.
You'll find that by far the best riding in VT, and the best way to see the non-tourist / fake part of the countryside is to stay the heck off the major routes, all of which are numbered. Going the way less traveled where the 'unnumbered' roads are a mix of dirt and pavement often has much bigger rewards. As an example, the Bayley Hazen military road (
http://www.nvda.net/transportation/pdf/BayleyHazenIntro.pdf is a bicycle route that I ride on my motorcycle) has a history going back to the early 1800s through eastern and northern VT, and takes you through some stunning areas, including Hazen's Notch at the end (the Notch conditions and difficulty documented on my website above).
I've started to cover all the routes suggested on
http://www.nvda.net/transportation/bikeped.html, this is through some of the most scenic and remote parts of Vermont.
As far as vehicles go, I'm not sure that a Mustang or Mini is the best option, as some of the most interesting and challenging areas in NNE include a little dirt road.
Lastly, if you're interested in Montreal, give Quebec City a thought instead. The old walled city is a World Heritage Site. One of the oldest settlements in North America, and very scenic, very French.
Montreal is much more cosmopolitan, and I'm pretty sure your wife will thoroughly enjoy the place.
I've had the best (and most expensive) meal of my life in the main restaurant of the Chateau Frontenac, which has stunning views of the St Lawrence. North and east of Quebec City the scenery isn't bad either: the Cote Norde east of St Anne along the St Lawrence through to the Saguenay Fjord is stunning in fall.
Quebec City is only about 3.5 to 4 hours from the I-91 border in VT on the major routes, or you can take in some Canadian French country life on the back roads. It is relatively flat, but still very interesting to see. One of my favorite towns to spend a night is Lac Megantic, and I have a ride most years (snowmobile and motorcycle), staying at the L'eau Berge inn (sp).
Have fun!
