Use the "Hover Bike" trick, then go into the first freestyle track. Go to
the end of the track with the tower, turn around and go full speed into 
the center ramp. Once in the air, hold [Ctrl] until you land in the stands.
Turn to the right and go slowly out the corner of the stands. Once you leave,
hold [Ctrl] until you land. You can now ride around outside of the stadium.