Hi, Beach Bum! It may depend just how close to Tokyo you’d like to be. I think for the most part people go to beaches in Kanagawa like the Enoshima area. I’ve personally been there as well as places like Chigasaki or Oiso. They’re general pretty fun but not that pretty to look at.
If you are okay to travel out to Izu in Shizuoka, the beaches are much more beautiful. Hope this helps!