I realize that this is hardly traditional HN fair, but watching amazed me. I had no idea that bubble rings could exist, or that dolphins would play with them like this.
Birds, for example, learn features of their songs from other songs they hear. Over time, these features accumulate in a bird's repertoire, and even develop into a dialect for an area. See http://en.wikipedia.org/wiki/Bird_song#Learning
Another example in birds is the behaviour of crows in Japan, who have learned to crack nuts by leaving them in traffic at intersections, waiting for the cross signal, then retrieving them. See http://www.youtube.com/watch?v=BGPGknpq3e0
This is the first example I've heard of a "play" behaviour being learned, but what can I say? The life of a captive dolphin would be boring; a new way to pass the time would be a useful thing to know.
(I don't own a dog, and don't have much experience with them, so take my comparison with a grain of salt.)
She was batting it around, clearly playing with it.
(Unlike other seals we ran into that day, she was uninterested in people. She reacted to us when we swam up but soon went back to playing with her toy and ignored us.)
Animals play.
They also play "hide", which often consists of putting their head behind a rock and peeking around to make sure that you're coming by. Yup, they leave their huge body exposed.