I did a ride from my home in Western Mass up through the southwestern corner of VT along the NY/VT state line up to Burlington and the Hero Islands. It was a very pleasant ride. There is a rail trail in the Poultney vicinity that was worth exploring, ending up in Castleton. Burlington is a very cool place to visit with a good bike trail that leads to the Heros but the causeway was closed when I got there due to recent flooding. If it has reopened and its little ferry is still running when you go I'd recommend it as a destination. The route also went through Middlebury, home of Middlebury College, which is another bike friendly town.

+1 on avoiding Rt. 100, the shoulders are pretty narrow in many places.

On the eastern side I've also ridden Rt. 5 along the Connecticut River from Mass to Canada. That is a better road for cycling than 100 and has many interesting places to visit such as: Brattleboro, Putney, Windsor (birthplace of VT), St. Johnsbury and Lyndonville. If you don't mind crossing the river on occasion into New Hampshire, Hanover, home of Dartmouth College, is another good bike-friendly town to visit.

On one of the above rides I returned from Canada via Rt. 108 and rode through Smuggler's Notch from north to south. Can't say how crazy that would be with foliage season traffic but it was a spectacular ride with a loooooong climb and a crazy fast descent down into Stowe which is the ski capital of VT.