There are plenty of places (especially around Burlington and the water-lined northwest) that rent canoes and other boats that are dog-friendly. You can pack some food and water and make a day of it or just paddle around for a while to get some exercise, get a new perspective, or just float.
Harpoon Riverbend Taps and Beer Garden is exactly what it sounds like. Certain days in summer they have Yappy Hour from 3-6 pm where you and your dog can come lounge in the beer garden, socialize, and play dog-friendly games while helping donate to a great cause for paws!
Check out 802 Disc Dogs, a club for dogs and discs that offers workshops, practices, and events, which is perfect for those who just want to give it a try, or tenured athletes.