Hydrangea is a typical summer season flower often used in ikebana. Hydrangea is native to the Japanese islands and has a long history in their culture. In the rainy summer season visiting temples with nice Hydrangeas, Ajisai in Japanese, is a popular activity in parts of the country. But the old Japanese version of Ajisai used in traditional ikebana arrangements is much more humble than the Dutch cultivars seen nowadays. The Western style Hydrangeas with big flowers masses, however, are very popular in modern ikebana. Have a look at this video from Seattle based Sogetsu ikebana teacher Megumi Schacher.
There are more nice ikebana arrangements on the website Ikebana by Megumi.