Our buddy Iris introduced us to Magnetic Hematite stones in 2009 at a child care conference in Idaho. If you don't know about them they are a naturally magnetic iron ore that look like that liquid metal dude from the Terminator movies when polished. Iris told me the kids in her child care program loved them so naturally we had to find some and try them out. The children in our program have had a ton of fun with them and call them "magic rocks". They have a nice magnetic pull, but are not strong enough to pinch little fingers.
Check this magnet board ideas blog post if you're looking for ways to support magnet play.