The hidden Pineapples in Angry Birds: Rio can be found in the first world of the game, Smugglers Den. The Pineapples are usually hidden in crates with labels on them, but also can be out in the open, or behind other objects. A good way to find out if a level has a Pineapple is to pinch the screen with two fingers to fully zooom out.
These are found in the second world of the game, Jungle Escape, and are trickier than the pineapples. The can be in boxes, behind bushes, or even behind leaves.
These are found in the third world of the game, Beach Volley.