Take the students outside and have them use their eyes to find things that are not man-made. You can narrow this down by personalizing it to the particular season or lesson. For example, the children could look for things that are green, or things that represent autumn such as fallen leaves or acorns and nuts. Tell the students to gather one heavy item (rocks are good), one light item, and some leaves. You can incorporate a discussion on the sense of touch by talking about how the items feel.
Return to the classroom and talk about "heavy" and "light" with the children. Set out some buckets about half full of water, and have the children test to see if their heavy items float or sink, and then have them test if their light items float or sink. They may be surprised by some of the heavy items' floating, such as apples. Explain that just because something is heavy or big doesn't mean it will sink, because some things have a lot of air in them.
Hold up the leaves. Have the students look closely at their leaves. Show them the veins on the leaves and tell them that those are tiny tubes for carrying water so that the plant or tree can grow. If the leaves are from deciduous trees, explain about the life cycle of trees. Then, create an art project from the leaves that the children can take home, or that you can display in the classroom. (For more ideas, see Resources.)