One of the great festivals I've had the good fortune to experience was a Ships Festival. Large old fashioned sailing ships came into the Dejima pier and all around the area was food and music. The ships themselves were incredible, I didn't realize how HUGE they could be.
During the afternoon we walked around and looked at all the ships, and watched dance performances. The festivals bring in a lot of people from all over so it was really crowded. In the late afternoon we met up with some friends and decided to go have kaiten sushi. This type of sushi consists of a bunch of little plates on a conveyor belt and you grab whatever you want and are charged by how many plates you have. It can actually be a really cheap way to eat sushi and you have a lot of options.