When Tokyo people are in the mood for a summer BBQ they head to Tokyo's riverside parks (e.g. Futako Shinchi). Such locations are popular in the summer. There always seems to be a party going on — night and day, rain or shine.
Families and university students are big summer BBQ fans. Parties by the river are inexpensive.
BBQ parties are completely legal (i.e. public alcohol consumption). Riverside parks are tolerant of the massive amounts of garbage the BBQ people create (as long as everyone follows the posted rules).
Family parties are in the afternoon and usually involve 5 - 20 people.
University student barbeques sometimes go all night and usually involve 20 - 100 people. In some cases, parties by the river attract up to 1000 people and feature proper DJ equipment.
Residents of areas such as Futako Shinchi are tolerant of the night and day sounds of parties (especially fireworks). In the summer, tiny Futako Shinchi station is overrun by party people waiting for friends. If you attend a BBQ, try to be respectful in area neighbourhoods (they put up with a great deal).
Popular BBQ items include thinly sliced pork and beef, fish, mochi and vegetables.
Summer BBQs can continue even in the rain thanks to the many bridges spanning Tokyo rivers (e.g. the Tamagawa).
As you might expect, much chaos ensues and fun is had by all.