I think my favorite fun thing to build would be slides!
I work at a place where everyday we build things geared toward having fun. It's an imaginative mix of playground, funhouse, living sculpture and architectural museum all made out of unique, found, reclaimed and recycled items.
Nearly everything we construct has an element of exploration - we've build all kinds of tunnels,hideouts, caves and climbers, but the one thing that is always a hit are the slides!
I mean, once you've clawed and climbed your way to the top, don't you want to get to the bottom as fast as you can to do it all over again?
I'd been toying with the notion of writing something to answer all the crazy questions I get and then I saw this post: Has-anyone-built-a-playground-slide
i guess it helped spur me into the notion that others might find a primer on slide building useful.
Step 1: Things You May or May Not Need
I build crazy things for a living, and that said I may have tools and or skills that you don't...
This doesn't mean you can't build a really cool slide and have an awesome time doing it. The materials you use will greatly affect how you construct it and what sort of tools you'll need to do it and of course the skills you'll need to do it.
I'll try to spark your imagination and set you up with the goods you need to get the mission accomplished and help you make it safe to boot.