It's the first day of fall, or something. And after August is finished, the beaches get all deserted. Hardly anyone goes swimming. The season is over.
99 Li Beach, or 九十九里浜, in Chiba, is the nicest beach I've been to in mainland Japan. It's long (99 li is equal to 66 km.) and broad and white and there are waves and there's plenty of space to stretch out in.
Brownsfield Organic Farm.
Although if you're going to Zushi or Shimoda anytime soon, I wouldn't sneeze at the chance to go to either of those beaches.
Quick, before summer really IS over.