1. Go for walks
2. Make "nature mobiles" with things that you find around your neighborhood and hang them from the trees.
3. Bury a time capsule
4. Start a garden (vegetable or flower)
5. Stare at the clouds.
6. Build a tree house or fort
7. Find some friends and play bocce, can jam, frisbee, or whatever you want.
8. Build a swing off one of the trees with some rope and a piece of wood.
9. Find a dog/cat/rabbit/pet turtle and take it outside.
10. Meditate. Find your inner you.
11. Take photographs of your neighborhood.
12. Think about geo-caching.
13. Build some bird houses out of wood or recycled materials and put them up in the trees.
14. Write poems that are inspired by completely mundane things in your yard (ode to a puddle)
15. Lay out a blanket on the lawn and read in the sun.
16. Draw with chalk on the sidewalk or driveway or patio.
17. Set up a treasure hunt. Leave clues around the neighborhood.
18. Buy some bubble solution or make your own out of dishwashing liquid and water. Blow bubbles.
19. Fly a Kite
20. Fly a paper airplane
21. See if your parents can get a chimnea, so you can roast marshmallows!
22. Build a hammock
23. Make a tent. Camp out in it.
24. Try to make a solar oven, and cook something. Force a family member to eat it (hot dogs work well.)
25. If all else fails force your parents to come outside and play with you. That'll teach them.
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