Fly a kite.
Play outdoor games like freeze tag, Red Rover, hopscotch and Kick The Can.
Color with sidewalk chalk.
Visit state and national parks in your area. Many offer great family friendly activities.
Go on a nature expedition at a local park, in your yard, or around your neighborhood. Bring along binoculars, a magnifying class, and a journal (young children can draw pictures of what they see) and teach your kids how to observe, enjoy and appreciate nature without disturbing it.
Make a nature journal. Have your kids decorate a notebook they can carry with them to note what they observe when they're outside. Even young kids can keep a nature journal by drawing what they see.
Explore the wonderful world of bugs. Check your local library for books about bugs and then head outside with a magnifying glass to see what you can find and identify.
Learn about birds. Check out some library books or use the online bird guide from the Cornell Lab of Ornithology. Then get outside with some binoculars and that nature journal and see what you can find. You can even take part in the Great Backyard Bird Count than takes place every February.
Grow a garden together.
Build and/or decorate a bird house.
Set up an outdoor scavenger hunt.
Play in the snow. Build a snow man. Make snow angels. Mix food coloring and water in spray bottles or bowls and snow paint.
Build a sand castle.
Have a picnic (even if it's in your own yard).
Start an outdoor hobby together. The options are limitless – biking, skating, fishing, hiking, canoeing/kayaking, horseback riding, tennis, etc.
Build a fort. Use these simple directions from wikiHow or get fancy with these instructions from This Old House.
Go letterboxing. Letterboxing is an outdoor activity that dates back to the 1800s. It combines elements of orienteering, hiking, art, and puzzle solving. Participants search for letterboxes hidden in public places, such as parks, by following clues from a catalog or web site. These letterboxes usually contain a notebook and a rubber stamp. Finders stamp their personal journal with that stamp to record their find and then leave their personal stamp in the letterbox's log book. Your own local parks and recreation department may have a program or you can check out one of the many web sites available to help you get started. If you'd prefer a more high tech treasure hunt involving gps tracking, try Geocaching.
Learn about the solar system. Kidsastronomy.com has several great resources you can check out before heading outside to connect the constellations together.