Pre-arranged codes/signals where the signal contains none of the message.
e.g. selling a certain item/s at a certain price on ebay means a pre-agreed action is to occur, to be simplistic.
This does however require collusion prior.
I think you need to set the scope better for the question. Easily usable by average Joe? Most secure using available for all technology / infrastructure?
Simple way: Signal messaging
Elaborate: one time pads, lasers bounced off lunar reflectors
signal messaging and one time pads are essentially equivalent, the former being useful only for a few bits of information, and the later is able to use the whole bandwith
I'm sorry, I meant Signal the encrypted chat application produced by Open Whisper.