Below is from Game Informer:

Nintendo announced today that those who preorder their upcoming game Mario Kart: Double Dash!! will receive a bonus preview disc chock full of demos and other extra goodies.

The disc will contain:
5 mini-games from Mario Party 5
A playable demo of F-Zero
Fire Emblem content that contains unlockable weapons, items, and new music
A playable demo of Teenage Mutant Ninja Turtles
Playable Hoth level from Star Wars: Rogue Squadron: Rebel Strike III
A playable demo of Sonic Heroes, featuring two levels and three characters
New movies of 1080°: Avalanche, Kirby Air Ride, Pokémon Colosseum, Final Fantasy: Crystal Chronicles, SpongeBob SquarePants: The Search For Bikini Bottom, Harry Potter: Quidditch World Cup, and NBA Live 2004

Thanks to Tonykewv1for the heads up.