The hidden treasure items are the ones that only appear when you walk into the right spot and a question mark appears over your character. There are many of these throughout the world -- Almost always tucked into corners or dead ends. Be thorough when exploring, particularly in dungeons, and this should come during the game. Always walk into dead ends and corners looking for items!
The quantity of the item (for example, rainbow coins) can be changed by backing out of the treasure and going back in. Don't settle for a low amount of coins, just keep backing out and clicking it again to get a larger number.