Upon completing the first stage of Sonic Mania, the following two options are unlocked on the main menu, which offer more stages the further into the Mania Mode you go:
#SONIC MANIC GRATIS HOW TO#
Watch on YouTube How to unlock new modes and Extra secrets in Sonic Mania 20170815 - Egg Reverie flying in any levels instead of Super Sonic.19890501 - Does nothing, or unlocks all Medals.
(Thanks to reader lanerobertlane for the tip.)