How Do I Unlock...

[InFaDeLiTy]

The last 2 skill games, and last 2 bikes.

[KRIS DETH]

If you check the achievement guide stickied at the top of the forum it tells you how to unlock all tracks + skill games :)

For the lazy people

All skill games can be unlocked by getting 3 gold medals in the Beginner, Easy & Medium difficulties

As for the bikes, you unlock one every time you complete certain difficulties, and the final bike, you must get a gold on the second tournament