Hang coffee cans (or dollar store buckets) from tree at different levels, paint scores of 20 on lowest, 40 on next, 60, 80, then 100 on highest. Use small, handheld dry erase or chalkboards for scorekeeping.
"Backyard Ker-Plunk". I made this DIY Ker-Plunk game by upcycling an old end table I found on craigslist. We wrapped some chicken wire around the top, stapled it in place to secure, and voila! Homemade Ker-plunk! We got the wooden dowels at Home Depot and I found the ball pit balls on craigslist. But of course, you can get them at Walmart or Toys R Us. It's a ton of fun!