Zippered mesh bags (from the dollar store) for storing toys in groups (especially ones that go into larger toy boxes). The set stays complete, and you can just throw the bag in the dishwasher/washing machine to sanitize!
Magic Treasure Rocks- baking soda, food coloring, and add water slowly until baking soda is moldable. Hide treasure inside of it. Let kids spray or drop vinegar on dried ball and watch it fizz and fall apart until treasure is revealed.