The town of Zushi (逗子) is one of the most famous and popular summer day trips from Tokyo, due to its pleasant beach, Zushi Kaigan.
The beach became famous a few years back for all the wrong reasons. Being near some of the US military bases it attracts a diverse crowd, including a lot of gaijin - whose partying, drinking beaching, presence caused some complaints to the powers that be.
The beach bars in summer are pretty good (seeing as you can’t bring your own this is a plus), and the sunsets are a nice experience, if you don’t mind sharing them with swarms of people. with Fuji views across the horizon on clear days, the little jungly cliffs surrounding add a nice tropical feel.
The waves are subtle and the sea is usually calm but it gets a lot of breeze, meaning Zushi Marina, a little south of the main beach, is always busy, and the swimming bay is popular with paddle boarders and windsurfers. Watch out!