Do a Google search for "recursion" and look in the "did you mean..." box. If you don't know what recursion is, just look for a definition and you'll understand why this is funny.
Also, try searching for "ascii art" and look at the Google logo in the top left-hand corner.
My favorite Google trick is searching for "google gravity" and hitting the "I'm Feeling Lucky" button. It isn't really all that fascinating but it is fun to watch.
And if you just want to sit back and play pacman on Google...
If you type in "Let me Google that for you," click the first one, you can type in anything you want and search it. Then you just have to copy and paste the url and it will search what you wanted to by itself. [url=http://lmgtfy.com/?q=Isn%27t+this+so+coolHere's an example[/url]
Click on this link. It will send you to a fake google page. Click once, the 2 O's will dissapear after 5 seconds. Click again, they reapear after 5 seconds. Click a 3rd time and it sends you to the original google. Have fun tricking your friends or amuse yourself by clicking the centre of the page then rubbing your hand slowly across the O's and then take them off. Abrakadabra, dissapeared.