To get unlimited ammo, hit 50 Barrels during Challenge 18.
To get unlimited health, hit 179 barrels during Challenge 17.
To get unlimited turbo, grab eight Zappers during Challenge 4.
To unlock all cars, hit 11 Bonus Stunt items while completing Challenge 8.
To unlock all Items, hit 30 Barrels during Challenge 7.
To unlock all worlds, find the Hidden Key in Challenge 18.
To upgrade the laser, finish Challenge 18 on very hard difficulty.