Sonic Mania is a treat for classic Sonic fans of all ages, but those that fiddled around with the original games & Knuckles will remember the debug modes that allowed you to mess around with any stage and completely ruin its subtle layout. Thankfully Sonic Mania delivers once again, as the classic stage select screen and debug mode reappears, albeit in a more restricted manner.
In order to access this mode you simply need to hold the B and Y buttons down on your controller of choice on the title screen before the 'Press Any Button' prompt appears. Then once it has, continue to hold these two buttons and press the Plus button. You'll be catapulted into the stage select menu where you can choose to play any stage the game has to offer.
Once you're in a stage if you press the X button you'll enter debug mode, which allows you to fly around freely and place various objects such as Rings, enemies, and Item Boxes. Objects can be cycled through with the B button, placed with the Y button, the X button returns you back to being Sonic, and the A button cycles through the different Item Box variations.
It's a lot of fun to just mess around with, but we would naturally recommend you play through the game normally to keep everything excellent a surprise for yourself. Once you've done that though, the game is your oyster as far as we're concerned.