Squirrel #4: Ski or Ride
Stop when you reach the first checkpoint, then reverse to get a better run up at the rocky ramp ahead.
Bailout from the top of that ramp to launch yourself into the open cable car cab above.
A cutscene will now trigger and the squirrel appears.
Squirrel #5: Shear Pressure
When you reach the final jump, drop down onto the chimney which will explode and launch you into the air.
Bailout on the way up to fling yourself over the finish line and into the crater beyond. If you don't make it over the walkway you'll need to restart the track to reset the chimney for another attempt.
After clearing the walkway you'll land in a squirrel-filled crater.