PPs have covered most of it. I do want to add a couple things.
This has great calendar listings: http://www.kidsvt.com/
If you are traveling via I-89, you could break up the trip home by stopping at the Montshire Museum in Norwich, VT. http://www.montshire.org/ We did that many times while we were looking for houses up here.
April is kind of a tough month. The weather could be nice (ie warm, spring-like) or still cold and places like Shelburne Farms won't be open yet.
oh! and B&J's free cone day is April 9 if that happens to correspond with your visit.
marie
mama to
DD1 9/03
DD2 12/07