My guy would sit and play with a big tub of beans for an hour. I'd give him some measuring cups and a bucket, and he'd put the beans in the bucket, dump them back in the tub over and over.
Also, he's always loved playing in the bathtub, so I bought a cheap can of shaving cream for him to put on the walls of the shower, and he could draw stuff in it, wipe it away and start over. That kept him pretty well occupied, too.