If it’s a sunny and warm weekend afternoon, you can count on there being a crowd lazing around the large grassy expanse that is Yoyogi-kōen. You'll usually find revellers and noisemakers of all stripes, from hula-hoopers to African drum circles to retro greasers dancing around a boom box. It’s an excellent place for a picnic and probably the only place in the city where you can reasonably toss a Frisbee without fear of hitting someone.
During the warmer months, festivals take place on the plaza across from the park (see website, in Japanese, for a schedule). Cherry blossoms draw huge crowds and parties that go late into the night.