In the Plantation level, go inside the "Rest Area". Then, go to the bottom where the water is located. Move left through the opening in the wall to reach the "Inner Wall" outside area. Booster 2.0 is required for this location. Keep moving forward until you find a door at the bottom left with a wall between it. Go under the wall, and boost up until you reach the door. Use it to enter a "Secret Grave" with a large "Prinny" statue at the center. Examine the sparkling object to get the Prinny Cap.