This is pretty cool. Dolphins have been observed hitching rides on the back of humpback whales in Hawaii, and sliding down their backs, for what seems to be, fun! Playful activity between species like this is something of rarity, which makes this story pretty special.

The video is from the American Museum of Natural History, and this behaviour was documented in the journal Aquatic Mammals by Deakos et al. (2010). Animal Wise has a review of the paper here.