Keep in mind scenic interstates are divided highways so everyone is going in the same direction with 2 or 3 lanes of traffic. I definitely agree with suggestion to take a bus some place away from Boston to rent a car where traffic is less congested. On country roads, sometimes I tend to drive slightly right of center but then I have to be very careful about any oncoming traffic especially when going around a blind curve. In our area, road edges can be broken down or have the most pot holes (here in NH frost heave season has started and pot holes can't be filled until it gets warmer). Sometimes I don't encounter much traffic between home and the grocery store in the next town. I do have to be careful about wildlife since flocks of wild turkeys can sometimes be standing in the road. The chair lift or gondola rides will probably require a car to get you to a ski area. I think Burlington VT has a lot of traffic in some areas so in cities you could consider taking an Uber to get around. Google maps vocal directions on your smart phone are a big help.

You'll be fine driving and it really is the only way to see New England. We went to Scotland last year and after the first day, when I swore my husband was trying to put me in a ditch, we did fine driving on the "wrong side of the road". Most of our roads in NH and Vermont, with the exception of the cities, are pretty quiet. The entire area surrounding Boston can be quite busy so I would suggest taking other transportation if you are going to Boston.

Look into taking at least part of Rte 100 in Vermont

One other town that is easy to get to from Boston via bus is Hanover New Hampshire. These buses like the ones mentioned by dfrost run between Logan airport and the towns in NH. They also stop at South station in Boston. They are quite comfortable and have wifi. The name of the company that runs this service is Dartmouth Coach. Hanover is a nice small college town and has some inns right in town.

I agree with others who have said you will be able to see a lot more if you drive. Unfortunately public transportation options are pretty limited.

Many tour companies come up to northern New England in the fall. I am not sure whether you could locate one for a few days out of Boston.