GBA eggs:
Certain eggs will unlock GBA mini-games when hatched. You'll have to download it into your GBA.

Game	Egg Location
Chu Chu Rocket	Pirate Island 2
Nights	Circus Park 2
Billy Hatcher Hyper Shoot, Easy	Dino Mountain 2
Billy Hatcher Hyper Shoot, Hard	Sand Ruin 2
Puyo Pop	Blizzard Castle 3

Extra emblems:
There are a few ways to get more emblems, including beating all missions with an "S" rank, collecting every coin and unlocking the whole Egg Gallery.

Play as Bantam Scrambled:
Beat Blizzard Castle Mission 4.

Play as Chick Poacher:
Beat Dino Mountain Mission 4.

Play as Rolly Roll:
Beat Pirate Island Mission 4.

Thanks to Revolution readers Bantam, FAmous Amos and live3!