Minecraft wiki entry for Planks, associated text: Mineshafts, where they are used as beams supported by oak fences and as a platform where air is beneath.
Plains villages, where they are used for various structures including walls, roofs, and floors of many buildings, as well as paths over water.
Strongholds, where they are used for floors and pillars in the library and storage rooms.
Woodland mansions
Shipwrecks