Use items from your household like ladders, brooms, newspapers, chairs and so forth.  Turn the kids loose and challenge them to make an obstacle course (or two!).  Remind them that they can go under, over, through, between, around and more for each item in the course.  Once the course or courses are set up, let the kids time themselves as they race through.  See who can improve their personal best! 

Take extreme close up pictures of textures and details around your house, then send the kids out to find the same items.  It's amazing how different the world looks when seen through a magnifying glass! 

Use a loop of string or yarn about 2 to 3 feet wide, and spread it out on the ground outside.  Arm the kids with a magnifying glass and challenge them to name ten, twenty, or even more living things in their tiny world.  With a notepad, they could sketch what they see, and with a field guide, they could identify many of the small creatures and plants. 

Give each child a shade from a large box of crayons and send them out to find something that matches their assigned color exactly.  Use the outdoorsy colors for outdoor play and the wild colors for indoor, rainy-day fun. 

Set up a blanket over a rope, table or chairs (indoors or out), and have a campout.  Make picnic food and use flashlights.  Sing camp songs and tell stories.  Pretend you are on a safari in deepest, darkest Africa or that you are camping beside the pioneers.   

Get a piece of paper and a pencil.  If the kids can't write yet, appoint a scribe to take down their words.  Have the first person in the group start the story and continue for a few sentences.  The next person adds whatever seems right.  Keep going and give everyone several turns.  If necessary, you can end the story yourself to bring closure to the characters and plot. 

Arm the players with rulers and a list of sensible sizes around the play area (like 5 inches or 20 centimeters, 2 feet or 4 meters).  See who can find items to match each size. 

Use graph paper to teach the kids how to make maps to scale, or just plain paper with a ruler.  Have them create a map to represent the backyard, then give them a 'treasure' (like a candy or a trinket) to hide in the area.  Mark the location on the map and see who can follow it to find the goodie. 

Use household dish soap, bowls or buckets, drinking straws, and paper clips.  Mix up some bubble solution with the dish soap.  Bend the paper clips into bubble blowers.  See who can make the most bubbles in the allotted time, or the tallest pile of bubbles, or the biggest bubbles.  Try the drinking straw and other ways to make bubbles to see what works best.  Put some food coloring in the bubbles to see if that changes their coloring.