Use the Tank:

To play as the tank, enter the variant name as "knat".

Want to Skip A Mission??:

During a T.R.I.P. mission, simultaneously press
and hold CTRL+SHIFT. Then while holding these buttons, type GETDOWN. You should hear Groove say "Get Down". At that point you should be attacked by all the other cars. Let yourself die and you'll skip to the next mission.

Play as the Helicopter:

To play as the helicopter, choose Phaedra Rattler
as your vehicle, then enter "retpocileh" as a variant name.

Use the Helicopter on Final Level:

At the vehicle selection screen for the final level, click on the helicopter in the upper left-hand corner.