My boys really like to play with rocks. We go on walks around the neighborhood and collect rocks. We make rock piles and hide rocks all around the house. We play rock-n-seek and we even load up the toy cars with rocks to transport to various destinations.
I have no idea what the big fascination with rock are, but they are free for the kids to play with and so long as no one is throwing them I am perfectly content to let my children spend hours playing with rocks.
Sometimes free is better… what free thing does your child absolutely love?