In Tokyo, you have two choices when it comes to the beach, Chiba or Kamakura.
Chiba is accessible by car and is pretty good for surfing, but it’s in the countryside. If that’s the kind of thing you’re looking for, Chiba might be good. Stay overnight for a full experience.
Kamakura is much more city-friendly and good to go if you want to relax or play with your friends or family. It’s easy to get there from Tokyo by train (about one hour).