You are in good position to drive south on the Champlain Islands. With an early start and not much stopping you can be in Burlington for lunch.

Boyden Valley Winery for a tasting (Cambridge VT)

Make a loop/day trip to Great Vermont Corn Maze in Danville VT. I would also include a stop in Hardwick VT to check out one of the businesses featured in Ben Hewitt's book, The Town That Food Saved. If your interested in Eat Local etc, you might find it a great read.

Look at old posts about Burlington. Recent post thought farmers market was great.

If you garden, visit Gardeners Supply and the Intervale section.

We stayed in Fairfax one trip so Burlington was a bit of a drive from there and you will be further north. Not sure how much driving you want to do.

Also in the Intervale section of Burlington, the Ethan Allen Homestead site is scenic and interesting.

Shelburne, south of Burlington, has Shelburne Farms and Shelburne Museum, both well worth visiting. I would probably pick just one rather than trying to see both in a day. There is also a vineyard in Shelburne.

From Highgate Springs you could also visit Montreal very easily if you have passports. If you prefer to stay away from major cities, the Eastern Townships area has vineyards, cideries, interesting historic sites. There are quite a few posts for either Montreal or the Eastern Townships if you do an Advanced Search on the Canadian Forum

Fort Ticonderoga

http://www.fortticonderoga.org/

http://en.wikipedia.org/wiki/Fort_Ticonderoga

There are also ferries on Lake Champlain and it might be fun to ride one of those ferries (never been meself):
http://www.ferries.com/

Eat at one of the restaurants on Church Street in Burlington.

http://www.churchstmarketplace.com/dining-on-the-church-street-marketplace/

http://www.tripadvisor.com/Attraction_Review-g57201-d106333-Reviews-Church_Street_Marketplace-Burlington_Vermont.html

Tour the Vermont Teddy Bear Company and acquire a new family member. That's how Paddington Bear joined our family.

http://www.vermontteddybear.com/Static/Tour-Essentials.aspx

Visit several of the other towns besides Burlington: Middlebury, Vergennes, Brandon, etc.