Have school age children form teams to find objects on a written list or complete a hopscotch that adds up to a specific sum. Ask children to find a specific object or something that is a certain color, is soft, is hard or shiny. For all ages, talk about the properties of the objects and how many of them there are.
Adapted from the website of the National Association for the Education of Young Children at http://families.naeyc.org/learning-and-development/music-math-more/10-ideas-get-you-and-your-child-exploring-outdoors.