Beach Adventure Time
Tokyo is relatively close to the beach, and there are a decent amount of beach parties and festivals. Many of the beach areas around the Shonan area attract huge amounts of young people who go and party, and occasionally there will be sponsored events by sports, music, or promotion agencies, making the party really wild.
Here is a blog that focuses on outdoor fun and events, and has some interesting insight into the outdoor Japanese party culture, mostly at the beach!

KurtisW
United States