All cars:
Collect 11 Bonus Stunt items while completing Challenge 8.

All worlds:
Find the Hidden Key while completing Challenge 18. The yellow key is in the Mines, across a trestle at the end of a tunnel.

All gadgets:
Intentionally hit 25 Barrels while completing Challenge 7.

Upgraded laser:
Complete all missions, then all challenges under the very hard difficulty setting. Note: You have to get a "A+" rank to unlock this cheat.

Unlimited ammunition:
Intentionally hit 50 Barrels while completing Challenge 18.

Unlimited health:
Intentionally hit 179 Barrels while completing Challenge 17.

Unlimited turbo:
Collect 8 Zappers while completing Challenge 4.

Timer speed:
Intentionally hit 13 Detour Signs while completing Challenge 15.