After earning a certain amount of stars, you will unlock the Arena. The Arena is similar to the challenges, except the layout of the checkpoints is random. However the rewards are better and it is a good place to earn stars.
Better turns
Learn how to boost out of turns. Boosting is done by holding down both sides of the screen. There is no limit to the speed boost. The fastest runs will require boosting from checkpoint to checkpoint. Depending on the car you are driving, boosting before your turn is completed will stop it and lock your vehicle going straight ahead. Try to start your boosts so that your turn ends just as you are lined up with the next checkpoint.
Recommended car
When first starting, you have the choice of the Higashi XR and the Volken Ladybird. The Higashi has lower grip but high top speed, and drifts easily. The Volken Ladybird has a lower top speed but higher grip. Select the Ladybird because it is easy to control. The car you did not select can be eventually unlocked. Each car has seven different colors, and each color has a different way to unlock it.