I would love this in my back yard and so would my kids.
Mount a chalk-painted board to the fence so kids can unleash their creativity outdoors. | 51 Budget Backyard DIYs That Are Borderline Genius
DIY indoor kids gym (easy and frugal). Heck, I'll just make one for myself when I get my own house.
planter baskets hold toys in the sandbox by Jackie from Happy Hooligans - full post here: http://happyhooligans.ca/2013/05/15/backyard-sandbox/
An entire DIY playground complete with rubber mulch!
Add a Bucket With a Pulley to an Outdoor Playset in a Few Easy Steps. This would be a good pulley experiment for the boys on their swingset
Simple DIY backyard toy- Pvc painted and mounted to 4x4 post. pouring pebbles sand, water, and more is so FUN! Makes fun sounds too
Sugar Bee Crafts: sewing, recipes, crafts, photo tips, and more!: Ideas for Kids
Climbing wall playground equipment