From Boing Boing:
This video from the American Museum of Natural History details two recent instances where scientists have observed a whale and several dolphins interacting in ways that are something we might classify as “play”.
More from Youtube:
Recent encounters between humpback whales and bottlenose dolphins reveal a playful side to interspecies interaction. In two different locations in Hawaii, scientists watched as dolphins “rode” the heads of whales: the whales lifted the dolphins up and out of the water, and then the dolphins slid back down.
Enjoy the cooperative play:
Related articles
- Dolphins and Whales Engage in Rare Interspecies Play (Video) (treehugger.com)
- Dolphins “Surf” on Humpback Whales Off of Kauai (damontucker.com)
- An Uplifting Dolphin Story. Literally. (io9.com)
Mental Flowers 
very cool