Two-year-olds love teeny tiny things AND they love to sort. Enter the lowly ice cube tray.
I used to let the kids collect all manner of junk on our walks, but then we'd have broken seed pods, dandelion seeds everywhere and gravel at the bottom of my purse. When I started to use egg cartons and ice cube trays things got a little more "organized" looking and we'd announce we were adding these specimens to our Nature Center under the stairs when we got home. With the compartments it all looks so official, like a museum.