Find all three stars in each GP level in single player mode to unlock a bonus. Here is a list of the bonuses and the levels they are unlocked from:

Micro Soccer - Beachside GP
Micro Marble - 911: Emergency GP
Big Heads - This Ol' House GP
Mirror Tracks - Toy Town Sci Fi GP
Concept Art - Jungle Jamboree GP
Color Tint - Hoedown GP
Bird's Eye - Perilous Places GP
Earn the Team - Graveyard GP