Use a button
Use a lever
Use a pressure plate
Use a tripwire hook and string
Use redstone to power the door
Use a redstone torch to power nearby circuitry
Use a daylight sensor
Use a detector rail with a minecart system
Use a pressure plate on the floor or ceiling
Use a command block with the `/setblock` or `/fill` command to replace it with an openable door or remove it
Use the door manually only if it is a wooden door, not an iron door