Today and tomorrow (Saturday and Sunday) is Koenji’s massive Awaodori street festival: 188 groups, 12,000 dancers, from the JR Koenji station to the Marunouchi Shin Koenji station. It’s one of Tokyo’s three largest summer festivals, and a lot of fun. A million people are expected.
TIP: Take the Marunouchi line to Shin Koenji station. Much less crowded so you can see dance better.