stage select
When sonic is going to spin towards you, press up,up,down,down,up,up,up,up and you will hear the chime sound. Shift the cursor down until you see the sound test. Go in, it allows you to select any stage except for some of them that are in sonic & knuckles.
debug mode
Do the stage select trick and highlight any of the stage and hold A and press start.
99 lives!
Go to lauch base zone and reach to one of those alarms and perfrom aand spin dash and hold it. Slowly, when the enermies come, they get hit and slowly, you'll keep getting a live after another.
Oil ocean music (from sonic 2)
At the sound test mode, go to the sound test and play this accordingly, 1,9,9,2,1,1,2,4 and it will be playing oil ocean music.
playing at night
Do the stage select trick and hold C and start a game.