Secret Items are can interact with it, press it 2-3 or 4 times will disappear and gives you a custom contraptions.
Only 4 were implemented, here the list you can found:
In Road to El Prokado, 6-12, the pineapple is at the cliff where the pig started the vehicle (construction area).
Use the Night-vision goggles or Alien Lamp Light, you can see the bottle under the bridge surrounded by the golds in Find The Statues. It’s on pigs left. It’s the hardest item to find.
The only skin of the pumpkin is in the main screen, near the ramp.
It’s in the credits, go to setting button on the menu screen, press the “i” button you must check all of them and press that crate. You”ll will have a scrap, achievements, skin of Item List (Randomly).