[Meredith AK]

I go to Montreal twice a year to visit a Customer and have gone both up 89 as well as 93. I find the 93 route the most direct and relaxing, switches to 91 in VT, then 55N once over the border and then 10 West straight into Montreal.
Should take you just over 4 hours of drive time, not inlcuding any stops. All highway and no small towns. Recommend stopping in Littleton or St. Johnsbury as you have a few lunch / dinner options there as well as topping off your tank because gas is very expensive in Canada (about 2X-2.5X what it is in the States). Once in Canada, you'll be on the road about and hour and a half before there's food off an exit.
If you go the 89N route, you'll run into countless of little towns along the way once in Canada, as well as lights and stop signs. Scenic, but slower going.

The border crossing at the end of VT91/CN55 is never crowded and vary easy to in and out. On your way back, make sure to stop at the duty-free store just before crossing back into the US. They have great prices on Canadian beer and a Crown Royal Limited Edition whiskey which is one of Crown's best offerings only sold in Canada.