On a hot summer day I went out to nearby Heiwajima in Shinagawa, Tokyo, to watch the dynamite boat race. Check out their English website where you can find out all the information about the race, the drivers and how to bet on it.
The entry fee was only 100 Yen and you could sit down or stand up and watch the race; it's very short but they have many races throughout the whole day. You can pay a little more and get indoor air-conditioned seats and there's many snacks you can buy on location as well.