Go into the Chao Garden. This has only been tried with Shadow, so you may want to use him. Go to the right of the rocks that hold the waterfall and go to the lower corner with the foliage (seen in the first screenshot). Jump and dash at the invisible wall created by the sky. Eventually, you'll jump through it and fly far, far away. If you go far enough, the camera will return to following you. You can then navigate with your control stick. Shadow will become separated into two flickering Shadows and you can float outside of the level, or go around it. If you approach the entrance to the garden, even from behind, you'll return to the lobby. If you go very far away and then return to the garden, you can land on it. Go look at your chaos - they will be underground, except for anything at the top of their heads! You can walk on them, but you can't interact with them. Just leave and return to fix them.
Mr. Missingno. has confirmed this works with Sonic.
Submitted by: Death Knight
Last sentence submitted by Mr. Missingno.