The fireworks display begins from 8 pm, and is launched from the left side of this photo. As you can see, it gets very crowded. Bring picnic gear and arrive early (either side of the river will do). Kawasaki Station, or at Rokugo Dote Station on the Keihin Kyuko Line.
This is Tamagawa River. To your right you can see the JR and Keihin Train Bridge. This side of the river is in Kawasaki, the other side of the river is part of Tokyo. In the furthest right side of the horizon is Haneda Airport.
You can see amateur baseball and football games on the grounds of Rokugo Dote Park. There are some golfing practice grounds both upstream and downstream, as well as excellent biking/jogging paths.
Other activities I have seen are jet skiiing on the river, and remote control flying of model aeroplanes, and picnic lunching.
Every year on 15th August, there is an early evening fireworks display, which is launched from RD Park near the left side of the photo. It gets very crowded, and people begin to bring picnic gear and gather on both sides of the river from around 3 pm!
The fireworks lasts for about an hour. It is LOUD - don't count on quiet concentration or naps during this time..