Beach in Kanto

Go to Enoshima. It is not the stunning one in Japan, however, it is the closest one to Saitama.

You can go to Enoshima from Shinjuku Station using Odakyu Line to Fujisawa Station and then transfer to Odakyu Enoshima Line which directly bring you to Katase-Enoshima Station.

There are a lot of events and festival in Enoshima during this summer, so I think it is the perfect place for you to go. You can look here for upcoming events in August:  http://travelenoshima.jp/coming.html