Ramps: Cut the sides off cardboard boxes and show your child how to set up ramps. Children will get excited to drive cars or roll balls down the ramps in order to test how far and fast they’ll go. Turn it into an informal science experiment by experimenting with steepness and different rolling objects. It's also fun to set up a race or obstacle course. Once they’ve played with the basic ramps, offer more varied materials like paper towel rolls and baking sheets to make your ramps more complicated.
Get Outside: Help your child explore the natural world. It’s amazing how exciting it can be simply to explore the woods and lawns on the side of the bike path: building with sticks, hiding behind trees, looking for ducks in creeks and turning over rocks. As a parent, I always expect to go farther, forgetting that just one small area can be exciting! Talk with your child about what you hear, see, and smell. Write it down in a nature journal for a literacy bonus!
Obstacle Courses: Set up an obstacle course for your kids. Show them how to run through it, and make it a fun, positive race. Can they go faster the second and third time? Then encourage them to make an obstacle course for you and a younger sibling. Even little kids can enjoy obstacle courses: https://www.youtube.com/watch?v=o9OoJ-Zg4QY
Create a Tiny World: Whether in the woods, on the pavement, or in your house (try using tape inside!), it's fun to help your children set up a tiny world. A giant inspiring example of this can be found here: http://babbledabbledo.com/giant-small-world/
She uses paint, but we often keep it more simple with chalk or tape. You can also try it out in the woods with sticks, rocks, and grass.