Unlock Super Sonic
Once you gather all 140 Red Rings, you'll be able to play as Super Sonic in every level. You'll need to collect 50 rings in the desired level, then tap on the Chaos Emerald that will appear near the top right corner of your gamepad screen.
Clear the game's first seven worlds and you'll unlock the secret Hidden World, which consists of four additional acts.
Lots of Lives
Advance through the Desert Ruins Zone 2 area, until you pass the final checkpoint ahead of the area boss. You'll find a beehive with three holes. Choose the lower one and you'll obtain four lives. Then you can kill Sonic and resume play from the checkpoint. It's easy to then collect four more lives, then die and repeat the process as desired, gaining three additional lives each time.