My son's favorite toy is the laundry basket full of animals. Basically he fills the empty basket with all his animals and pushes them around. Sometimes he gets in and we give him a ride. He will do this all day long.
Here's a picture with a box. He loves boxes too. He will put things in them and carry them all over the house. He tries to sit in them too, even when he sees boxes in public places like the library!