Sōgetsu is one of Japan’s leading schools of avant-garde ikebana – Japan's centuries-old art of flower arranging. Classes are held in English on Mondays from 10am to noon; book via the web before noon of the preceding Thursday. The class fee includes materials. Exhibitions are sometimes held in the lobby.
Even if you have no interest in flower arranging, it’s worth taking a peek in for the building (1977) designed by Tange Kenzō, and the giant, climbable piece of installation art by the revered Japanese-American sculptor Isamu Noguchi, which occupies the lobby. There's a cafe here, too.