Grand Isle County: See
The islands aren't exactly packed with things you can see in the traditional sense (i.e. I came to take a picture of this specific attraction). However, the islands are one of the most beautiful parts of Vermont and gorgeous scenery is the norm. Obviously, the islands are surrounded by Lake Champlain, which is quite scenic on its own. To the east, you can see the Green Mountains rising in the distance. Look southwest and you will see the Adirondacks meeting the shoreline.
Here are some specific things to see:
Chazy Reef Formation. Possibly the world's oldest coral reef.
Grand Isle Art Works, 259 US Route 2 Grand Isle, ☏ +1 802 378-4591.
Hyde Log Cabin, Route 2, +1 802-828-3051. Considered one of America's oldest log cabins and is home to the society collection. Open Th-M 11AM-5PM.
St. Anne's Shrine: Isle la Motte +1 802-928-3362. Visit this shrine marking the site of the oldest European settlement in Vermont. There is a visitors center and gift shop.
Vermont Shakespeare Company, Knight Island State Park.
The Lake Champlain Monster, Otherwise known as Champ, this prehistoric sea monster supposedly lives in Lake Champlain.