Summer Dance Festival-Hopping

Harajuku Omotesando Genki Matsuri Super Yosakoi and Koenji Awaodori are both 2-day dance festivals that take place on a weekend in late August — Harajuku Omotesando Genki Matsuri Super Yosakoi is from morning to late afternoon and Koenji Awaodori is from 5 to 8 at night so you can go to one and then easily commute to the other by train in 15 minutes. It's definitely worth checking out both. Asakusa Samba Carnival and Tamagawa Fireworks might take place the same weekend. Both are amazing (and free!)

Harajuku Omotesando Genki Matsuri Super Yosakoi

There are a few stages around Meiji Jingu by Harajuku Station — each dance team presents a very impressive and organized dance routine choreographed to traditional-pop fusion music. Very genki indeed.
Koenji Awaodori

Koenji Awaodori is at different spots around Koenji Station with processions of dancers and musicians playing traditional Japanese tunes. Of course there are lots of street vendors with fried stuff and drinks! Great atmosphere. Very crowded! Don't be discouraged by the crowds though... once you stand behind people for a while, some will leave eventually and you can take their spot!
Asakusa Samba Carnival

Do not miss Asakusa Samba Carnival, which probably takes place the same weekend on a Saturday!