[Note: this event is outside of Tokyo]
Way down in Kyushu in the middle of the Kuju mountain range you'll find an unusual music festival devoted to connecting with the mountainous surroundings and rediscovering the peacefulness of nature. 'Calling Mountain' is a two-day festival featuring music performances, workshops and even talk shows. There's also stargazing events and guided mountain treks. The workshops include a demonstration in turban-making and doll-making. The underlying theme in all of these eclectic activities is to get out of the city and reconnect with nature.
The DJ line-up includes Force of Nature, probably best known for their work on the 'Samurai Champloo' anime soundtrack. Access from Tokyo is a bit tricky, but you can check the website for the best highway bus routes.