That my new idea for the challenges. For me challenges is essentially fun. Actually challenges are difficult and boring. They are race challenges and for race challenges, we have many mode and grand prix. My main inspiration is achievement (in xbox 360) and trophy (in PS3).
The challenges are listed in order of difficult.
one stars -> really easy
five stars -> very difficult
Some exemples
Nitro is good: Use nitro for 20 sec (2 stars)
ice cream: slow down 5 enemy with the cake (1 star)
Around the world: travel 1000 miles (5 stars)
Energy economy: Don't use nitro and finish first one (2 stars)
Space race: Load the nitro bar in XR391 for launching the rocket (5 stars)
Slash: Fall in every game liquids (1 star)
...
Each challenges have a different icon (in this case is ice cream challenge).
And the trophy room (I used the concept of dakal's GUI)
For an exemple of achievement of a game see this (halo reach achievement) and achievement (wikipedia)
What do you think ? What's your opinion ? Do you like this kind of challenges ? Have you any idea of challenges ?
And 'Energy economy' sounds rather challenging 
