While holding your animal, go as close to your chao as you can without giving it to the chao. Then, just drop the animal in front of your Chao. If done correctly the chao will give the same animation as if you handed the animal to the chao, only you can pick the animal up again. You can also do this with chaos drives.
With two controllers plugged in and in Sonic's second stage (Metal Harbour), press Y in the second controller at any time. Sonic will get a board under his shoes and will be able to street board the level with it! However, you can't complete the stage with this board.
Collect all 180 emblems in the game to unlock the secret Green Hill Zone, accessed from the Stage Selection screen (a green island). This level is a recreation of the original Sonic zone from the very first game.