It is more practical to go to Nantucket as a foot passenger as there is only really one town on the island and you can walk around it, there is also a bike rental place near where the ferry docks.It is a very pretty town with well preserved old buildings and a lovely harbour which at this time of year will be chock full of some of the biggest yachts on the East Coast. Its nice to sit out on Main st in the evening and watch the world go by but things are not cheap on the 'Grey Lady'. Trip out from Hyannis is about 2 hrs by ferry or you can catch a 12 seater puddle jumper from the airfield in Hyannis flight out is about 20 mins( try and get the co pilots seat...great view).
The ferry out to the Vineyard from Woods Hole takes less time but the island is bigger ,more to do and see and you would need a bike or a moped to get around it. Edgartown is worth a visit and you can hop over on the Chappy Ferry to the island where Ted Kennedy met his Waterloo( nice beach out there). The other end of the island is referred to as 'up island' and there are some nice quiet lanes for cycling/walking.
If you are not into browsing for beads, well the Cape and Islands are renowned for their fishing, Bluefish or Sea Bass, what a fight when you hook one of those babies, and every sort of water sport is catered for.
If you are short on time ,skip the Islands altogether and stay on the Cape, visit Chatham and Provincetown instead. P'town is good for whale watching.