Kids are always willing to do a sink or float experiment in nature, because learning science this way provides for a sensory experience. It is always wonderful to let the kids discover things on their own, as opposed to teach them form a textbook.
You need a muffin tin and some water. Ask the kids to bring some natural objects: soil, sand, pebbles, leaves, rocks, etc. Then, place each of these objects into its own opening in the muffin tin. Add water and see what sinks and what floats. Discuss the concept of density and how things with more air in them float better.