Dolphins are one of the few animals that play all through their lives, even as adults. They tend to hunt at night, rest in the mornings and then play in the afternoon. They can play with anything from floating bits of drift wood to bubbles that they have blown. A common game is where dolphin blows a ring shaped bubble and another dolphin has to swim through it before it hits the surface.
Sleep is a great problem for any sea mamal as they have to breath and if you fall asleep in water then you could drown. That is why dolphins don't sleep. What they do is shut down half of their brains at night and then the other half in the day so that over the course of 24 hours their brain gets at least 12 hours rest. They use the morning to rest their bodies rather than their minds.
Dolphins use clicks to comunicate between each other as that is the only sound that they can make with a few exceptions. Resently a team of scientists have created a system where, a screen is put into a tank with some dolphins. It gives them an option of several different images. The dolphins know that each image means a different type of food, for example one might mean squid the other fish. The dolphins send out a series of clicks at the image that they want to eat and the screen records the one that they want. Only two dolphins in the world know how to do this. It is the first real comunication between humans and animals, even if it is only asking them what they want to eat.