This winter, I started filling up his bathroom sink with water and giving him a cup and a bath toy. He would spend a good 45 minutes playing in it.
Earlier this week, I found his new favorite pastime. I fill the tub up with less than an inch of water and let him stomp and splash and have fun. It's cheap, fast and fun. Because there isn't much water in the tub, he can't splash any out so there's essentially no clean up.
Why have I not thought of this before?