To unlock FRE vehicles early: at the beginning of Freeride Extreme, go to NW Oakwood and up to a blue door of a house by a phone booth. Right click to make the Action icon disappear. You can see the vehicles it unlocks by checking the menu.
Race short cut: after the 1st checkpoint, turn left and follow the path. After a checkpoint message appears, press Num 0, then continue from the start of the race and repeat.
Monster cave door: in Freeride Extreme, you can go past the end of the RR track in NE Hoboken/S of Salieri's warehouse and jump from an engine hood into the mouth of the cave that looks like the mouth of a monster. Inside, the door on the left is locked.
To unlock the tunnel the door opens to, turn toward town from the cave entrance, go to the road, go S, take the 2nd right, and go right over the lot to the door of an ironmonger's shop. Right click when the Action icon appears.
Return to the cave, get over the barriers, and go right along the outside of the cave. When an Action icon appears, right click to cause a door to open to a tunnel that leads into the cave.