Bikes
New bikes will become available with each new set of levels that are unlocked by completing stunts in each quadrant. Do not focus too heavily on bike parts. If you have a chance to get a new accelerator, get it to help your race times. However new bikes come with them automatically; wait to unlock them before spending money.
Challenges
Various challenges such as getting the highest jump, landing in a specific area, or having the correct amount of speed coming off a jump will appear through indicators scattered throughout each level. There are also stunt-based challenges that can be passed by executing one or more flips by leaning your rider all the way forward or back while airborne.