Insect hotel - Sustainable ideas including an explanation of the materials used, which insects they provide a habitat for and why you want that insect. This would be perfect to have as a project to build with a youth program
Use an old tire to create a small backyard pond. My kids, 3 sons, have been helping with yard work and planting. They keep asking for a pond. I think this is a great DIY project to do outside with them. Looks simple.
Clothesline "Sculpture" - I can envision putting this at the end of the deck. For evening outdoor gatherings when not in use, put up strings of lights and food/drink table underneath. Or a nice canopy with climbing roses, clematis or other vine flowers.
Log Bird Feeder - 23 DIY Birdfeeders That Will Fill Your Garden With Birds (my note: I haven't looked at all of these, but at least one is a terrible idea. The window birdfeeder -- birds could fly into the glass and hurt themselves.