Fill it with water, gas it up to ~30psi and you have soda water on tap

But that's not the best use of it - end of an evening disconnect your other keg lines and hook them up, run the wter through and you've cleared your lines of beer and helped keep them clean - now THAT'S a good use! And no more wasted beer in the lines either or crud build up. Simples but effective and better than an empty one standing idle.