A couple of interesting things were stuck onto the Sonic & Knuckles Collection CD.
First is the full version of the Sonic Screen Saver. You can install it by bypassing the installer for the game itself and exploring the CD.
Second is a lot of artwork for the games. That's nice and all, but as nice as...
The best bonus on the disc: the ENTIRE Japanese Sonic CD soundtrack! YES! It's there (albeit in rather low quality WAVs), and it's worth it just to hear what U.S. gamers missed out on: the best Sonic music composed by mortal men.