I didn't see any beaches anywhere handy on Corfu, so can't comment; maybe with a rental car you could drive to some other part of the island. Mykonos has lots of beaches so I think you could either walk or take a taxi, probably don't need a rental car. Taxi cabs will be abundant at every port of call except Santorini. Take a look at the "Ports of Call" section of the message board on Cruise Critic to see what others say about their experience, too.