Our trip was for 2 nights and 3 days. We visited Yokohama on the first day and Kamakura on the last day on the basis of consensus between my wife and me. On the second day, however, our schedule was determined by my 3 kids: Tokyo by my daughter and Saitama by my 2 sons.
Some may say, "Tokyo is fantastic, but why Saitama?" I basically agree. There are so many sight seeing spots in Tokyo. My daughter requested her mother to take her to Tokyo Disney land and a town called Harajyuku, which is representative of street culture and fashion trends in Japan.
My sons told me, "Dad, Don't you know Saitama has the world largest UFO catcher center?" UFO catcher is kind of an attraction, in which you control a small crane suspended from the ceiling of a clear box that is full with items, for example, figures, toys, candies, and catch what you like and carry it to the exit of the box. If you success, you will get the item.
We actually went all the way to the amusement center. It took an hour by train from Tokyo to the nearest station. We then took a taxi to get there. Look at the picture above. My kids are about to enter the game center. I am not sure if they thought the amusement center was worth visiting, but they spend a few hours without eating lunch.
I am very happy with the short trip because the process to get there was like an adventure. So far, no guide book seems to recommend it, and I hope they won't from now on. Very memorable summer vacation!