Cheat mode:

Start Championship mode on the Coventry Cove track. Find the shortcut near the barn with two haystacks. Intentionally drive into the haystack closest to the track and hit the box hidden inside. The word "Groovy" will confirm. Now finish the race. A "Cheats" option is now found on the Options screen.

More Beetles:

Beat the Novice tracks to enable the Lightning Beetle, the Number 8 Beetle and the Racing Striped Beetle.

Even More Beetles:

Beat the Advanced tracks to enable the Lightning Beetle with a spoiler, the Number 8 Beetle with a spoiler, and the Flower Beetle.

Alien Beetle:

Beat the Professional Championship to enable the Alien Beetle.

Bonus Mode:

Beat all the standard Championships to unlock a Bonus mode.

Police Beetle:

Beat Bonus mode to enable the Police Beetle. As this car, press C-Left to use the siren and your opponents will pull over.

Hints

Breaking Out:

This is a glitch in the game. You can do it on any track it's the best if done on the ice level Mount Mayhem. Go around the track all three times, when it says final lap turn around and go backward through the level. When you get to the barrier that you can't go past, go to the far right corner and press (L) it will take you on the other side of the barrier, then head to the end. This makes it possible for you to go past the finish line and slide around on the ice.