Really busy beaches in the summer with bars and restaurants. Charming island with very authentic food stands and a secret private beach. High-class onsen with view on Mount Fuji. All these is loctated one hour from Shinjuku.
You probably have heard about the Buddha statue but you perhaps don't know that Kamakura is a lovely towm with a long beautiful beach, really popular among japanese surfers.
Certainy one of the best beaches in the Tokyo area and that is why the Emperor himself has a house just near-by. The summer beach bars and restaurants during the summer are also really chill and you can even reach a desert island by swimming.
Zushi would be on your way to Hayama but maybe deserves a stop. Quite few rules on this beach about alcohol or tatoos but the quality of the water is a plus and the Zushi film festival can be a good opportunity to go there.
Oiso beach is not the most famous one but has a lot of history and a view on Mount Fuji. The sea is wild and the waves are big, which makes it a top spot for surfers. Quite popular during summer, it deserves to be on your checking list.