I picked up a book at the beginning of the summer called Handy Dad. The idea was to pick a dozen projects that I could do with the kids in the coming year.
We did the water rocket on July fourth. I'm pretty sure there's video of it somewhere.
Both kids picked the climbing wall, which is really simple in concept, but the instructions tell you to buy preformed climbing handholds, which, while very cool, are also very expensive.
So I decided to make them myself.
I got a 4 x 4 fence post and I'm cutting 3 or 4 inch pieces from it, and forming them with a dremel. It takes anywhere from 2 minutes to a half hour to make one, depending on how elaborate you want to make it. Later today, I hope to attach the first batch of handholds on a tree down at the creek, and will post pictures of that, too.
This is part of a much larger project, including a rope bridge and a tree house. I expect that this will take at least until next summer. I'm really looking forward to how this turns out. I'm completely making it up as I go along, and have no idea what I'm doing.
I recommend this book, along with Fifty Dangerous Things You Should Let Your Children Do as project starters for you geek dads looking for ways to indoctrinate your not-yet-geek kids.