I'm a big fan on Freecycle & managed to get a load of pressure treated lumber. Free stuff is great. Free lumber is a miracle. So, my older boys (11 & 13) helped me build a new set of bunks for the boys' room.
They love their new beds & I'm happy because it's way more stable than the metal loft bed that was in there previously.
And, because I despise throwing anything away, I turned the loft bed into a greenhouse. I have to say, I'm happy with the results so far. We're still harvesting peppers, even though we've already had a freeze & my garden is now gone. My greenhouse may actually help me to not hate winter.
Our other big build was a chicken pen. Our city allows us to keep a few backyard chickens. The kids' old swing set frame was bent & no longer safe, but it makes a great frame for a coop. I also covered one end with a plywood roof, covered in shingles fashioned from aluminum cans. It's ugly, but functional. And I'm happy to have some birds again.