Here's one I know off the top of my head. If you play for 20 hours overall play time you will unlock one of the robot's

f you get gold emblems in all missions you get to play as super sonic.

If you play for 15 hours, you unlock E10000G. If you play for 100 hours, you unlock E10000R. If you complete all of Storm/Wave/Jet's missions (that includes SEGA Carnival and SEGA Illusion tracks), you unlock AiAi, Ulala, and NiGHTS. To unlock SEGA Carnival, go to "Normal Race" and play EX World GP. Play the "Hero GP" with any character and win with a Gold Rank and 50pts. To unlock SEGA Illusion, go to "Normal Race" and play EX World GP. Play the "Babylon GP" with any character and win with a Gold Rank and 50pts. With the new characters and tracks, you can do all the missions and unlock secret boards in the "Black Market." ^^ I know a little bit too much!