South of Tokyo in Chigasaki on the Tokaido line lies a convenient option for a day at the beach, or for the more sporty, a day in the surf. Chigasaki beach has very easy access from the station, and along the way are plenty of shops and restaurants.
A good solution for board rentals is the Southern Beach Surf House, located near the symbolic "C" sculpture on the beach (they rent out a lot of other water sport equipment too, see the website for pictures). Walk from Chigasaki station down to the beach, and then West for about 10 minutes until you see a pier. They even rent out barbecue sets, and of course have hot showers for when you finish up your day.