I got this idea from Melissa from Chasing Cheerios (I love her blog, J and I will be trying out many of her activities).
I set up a tray, a sponge, and a salt shaker of water. I showed J how to sprinkle the water onto the tray and how to wipe it up with the sponge. He was very engaged in this activity wiping and sprinkling over and over.
Ah, sweet sweet concentration! I actually managed to unload and load the dishwasher!