'Traditional' humidifiers look like this:
It's just a ceramic container with a hook that hangs over the radiator. Fill it with water, and the water evaporates as the radiator heats it.
You can emulate this with a dish or bowl hanging on your radiator. If you have the space the easiest option is to place a container on top of the radiator.
It was not uncommon to add scents to the water, like lavender; this supposedly helped combat the 'stuffiness' of the room.