A walk in the woods, nature park, schoolyard or even your own backyard provide sources for collecting plant samples and making wildlife observation. Children can collect leaves, compare shapes and vein patterns and try to match them to the tree variety. A child's own body can teach him about human anatomy and physiology. He can take his pulse, measure respiration, hearing and visual acuity and complete various physical feats while noting which body parts make it possible for his body to move in that manner.
Model flying devices and boats let children experiment with flotation and aerodynamics. Blowing bubbles is another way to spark discussion of what makes an object float. Watch the changing states of matter in action by spraying a sidewalk or outdoor wall with water on a hot day and measure how long it takes for the moisture to evaporate. Ask children to lean over as far as they can in one direction without falling or build block towers to discover what it takes for a person or a structure to stay balanced.
There's nothing like a nocturnal observation to get children to understand the mystery of outer space. Whether you attend a star party run by professional or experienced amateur astronomists or simply go star watching in your backyard, children can learn how to use a telescope, locate constellations, planets and other astronomical phenomenon. If cloudy skies obscure your view, try visiting a planetarium to make simulated observations.
Weather is an easily observable science with which all children are familiar. But setting up a homemade weather station facilitates a closer observation of weather processes. Rock collecting gives children a chance to observe and sort rocks and minerals by different characteristics such as color, hardness and transparency. Let children use a slinky to demonstrate the wave patterns created by an earthquake.