http://info.sonicretro.org/Sonic_2_XL

Even if you're not into fat furs, this game still presents a very interesting and challenging mechanic: onion rings. That's right, onion rings. All the gold rings in Sonic's world have been replaced with these oily snacks, and eating them works just the same as collecting a gold ring, but you also gain weight with each one you grab! So you have to balance your collection with exercise, and keep your weight just right so you can still run. Burning weight doesn't actually cost you rings, so you can still keep a high score, but not running them off will slow you to a halt, eventually immobilizing and killing Sonic.

The game is a very fun and fresh twist on the classic game we've all played to death, and I find it very unique and creative. Interesting mechanics like this are what I look for in mods and hacks, and this game does not disappoint. If you have a Sega emulator (or, God forbid, a Game Gadget), I highly recommend giving this game a try.