Take the family for a walk at a nearby park or go exploring in your backyard. Bring a bag or pail to collect interesting leaves, flowers, seeds, and stones that you find. You can play "Twenty Questions" as you walk. (One person picks something she sees and the other players have to ask questions until someone guesses what it is.)
Try Tree Spotting
Older kids can take a stab at identifying trees using a guide like Gina Ingoglia's The Tree Book for Kids and Their Grown Ups. If you've got 'em, binoculars can interest aspiring bird- and other creature-watchers.
Be A Spy
Find an animal or bug and see how long you can (quietly!) watch it without being noticed.
To create a terrarium, layer small rocks (for drainage) in a widemouthed plastic or glass container. Spread a few inches of dirt or potting soil and then add dandelions, little plants, and bits of grass from outside, all with the roots still attached. Water lightly and cover with the jar lid or plastic wrap and a rubber band. Place in a sunny spot and mist weekly.
Let the Sun Shine
Try making easy shadow prints from Let's Go Outside!, by Jennifer Ward: Place a piece of dark-colored construction paper on a sunny windowsill. Cover it with items from your walk. After several hours, the sun will fade the paper, leaving silhouettes of your finds.
Make a collage using leaves, flowers, or seeds. For inspiration, check out the work of artist-naturalists like Georgia O'Keeffe and Andy Goldsworthy at Morning-earth.org. For more nature-themed games and activities that you can sort by the age of your kids and the amount of time you have, click on the "Find Activities" tool at Naturerocks.org