Have all 32 Skylanders registered, then start a new game. The opening sequence will now feature all 32 characters in a large fight sequence.
Note: This trick requires two players. After completing Crystal Eye Castle, do not start the next level. Go to the Molekin camp, and have one player be Gill Grunt and the other any fire element. Have the Fire Skylander stand in the fire, and have Gill Grunt shoot the fire. Do not do this too long without collecting the money or else the game will freeze.