If you're interested in hopping over to Martha's Vineyard while you are there, I would recommend taking the ferry from Hyannis to Oak Bluffs. From there, you ride a short distance through the downtown area and a cool Victorian-looking village to reach a six-mile, totally flat bike path that will take you all the way into Edgartown via several public beaches along the coastline.
There are some tricky turns to look out for when you get to Edgartown; particularly one spot where EVERYBODY seems to turn right instead of going straight. If you get lost, ask somebody where the Edgartown lighthouse is.
While you're here, there are some good outdoor seafood places (vegetarian-friendly, too) to grab a meal. Also, a very short (but pricey) ferry will take you from Edgartown to Chappaquiddick.
Be sure to leave time to visit downtown Oak Bluffs and the waterside restaurants and bars before you return to the ferry!