When first starting, you can access the following power-ups.

Fireworks: Launch directly ahead to damage opponents.
Tiki Seekers: Similar to Fireworks, but they can lock on and home in on targets.
Oil Slicks: Pool of oil forms behind you to spin out opponents.
Low Gravity: Causes cars in front of you to bounce around.
Death Bats: Screeching fireballs that target the first place buggy and hit anything else in the way.
Spiked Tires: Increases your off-road driving temporarily.
Boosts: Speed boost.
Basic Shields: Temporarily protects you against projectiles.

Expend power-ups that you will not be using in order to make space for the reward given for hitting the next question mark. For example, use Oil Slicks when in last place, or Fireworks while in front to clear them out.

Save Basic Shields for the end of a race. They are a very useful counter for when AI drivers try to hit you with power-ups just as you close in on the finish line.