Pick up some of the green powder at the site where the trailer crashed. Use it to defeat the woman with the chain saw. The chain saw can defeat any other biker, including the blind goggle types.
Easy wins
Press [Left Shift] + V while fighting the bikers on the Old Mine Road or during the destruction derby.
New engine
Kill all the bikers on the old main road, then turn right twice, left six times, then right. You will get a new motorbike engine.
Play with a knife
After you fix your bike all up from the crash and go back to the "Kick Stand" (the bar you start the game in), there will be a Semi in the front. Enter the bar, there will be a man at a table playing with a knife, stabbing around his fingers, and sometimes hitting his fingers. Talk to the man (Emmet) at the table and say to him "I know how to do that". He will give a remark. Keep bothering him by saying "I know how to do that," and "Let me show you how to do that". Eventually he will say "All right, if it'll shut you up". You can now play the same game with the knife and your hand, and can even give yourself cuts. You may do this as many times as desired.
Shredding bunnies
Steal an extra box of bunnies and save them until near the end. When you are on the front of the truck speeding down the highway, open up the engine compartment. Get the box out of the inventory and put it on the engine fan. Ben will put the rabbits in the blade one at time and shred them. Also, in his rough voice he will say "Cool".
Skip Destruction Derby
Press [Ctrl] + C.
Vehicle display
Stop moving do not touch any objects for a few minutes. A screen that displays and rotates various vehicles will appear.
Chain saw...