Aim for half. You'll get 10 if you're intentional. You'll remember all 10.
Sleep outside one night
Run through a sprinkler
Catch a firefly
Make popsicles from juice or yogurt
Watch a sunset all the way through
Read a whole book in one day
Go to a farmer's market and pick one new food
Bike somewhere you've never biked
Swim in open water (lake, river, ocean)
Make s'mores over any flame — stove counts
See fireworks
Plant something from a seed
Stay up past bedtime for a good reason
Write a letter to someone
Climb a tree (safely)
Wake up early to see the sunrise
Make and sell lemonade
Go to a library event
Pack a picnic and eat it somewhere new
Do one thing that scares you a little
Next summer, make them write their own.