Collecting Chaos Emeralds
Get 50 Rings in all 6 Zones in either Act 1 or Act 2 and a Special Stage will appear. To get the Emerald in Game Land, you must win the race against the CPU with 50 rings.
Nega-Mother Wisp
Complete Terminal Velocity with all emeralds. After the credits are over, return to the game. After a quick cinematic, and the final stage with Nega-Mother Wisp will be available at the very top of the world map.