Sometimes the best toys are cheap ones; my kids love to build card houses out of flash cards. We recently bought a few packs of index cards and small labels and let them go to town with them. They loved building houses (and tunnels and train sheds and tents) with them, then putting their toys inside. I loved re-arranging their little figures into odd scenarios, like the possible crime scene, above.
Read more about office supply crafts in my post over at TravelingMom.