It was conceived as a "kids playspace" that they could decorate as they wish, spill things without consequence, and generally have a haven from the concerns of the adults relating to not breaking things in the house. As such, I had to continually be reminded throughout construction that "This is a CLUBHOUSE not a GUESTHOUSE!!" because I'd get wrapped up in some silly cosmetic detail.
Our local museum center has a few spots that are outfitted to resemble a clubhouse or treehouse and I keep hoping that our kids will use found objects to "decorate" thier space in a similar, eclectic, fashion. Unfortunately, they won't leave the yard to go find things.....
In general, you are looking at an elevated deck topped with a small shed. The roof of the shed accomodates a garden that serves a purely decorative function, although there is some small stormwater runoff control benefit. It is high enough that I will not be getting up there to harvest vegetables on anything like a regular basis. Perennials and groundcover to suit your climate are probably the best. We haven't decided much yet.
Once the paint went on, I had a new concern. I'm afraid Ronald is going to come after me for trademark infringement. My neighbors keep knocking on the door and asking where their orders are. Well, the ones who are still speaking to me anyway.....
A word on legality: In my city, a child's play structure does not require a permit unless it is over 12.5' tall (oops) or enclosed (darn).
Step 1: The plan
If you aren't familiar with Google Sketchup, it is a free 3D design tool provided by *suprise* Google. Can't find it? Just Google "sketchup". I found it to be very intiutive and easy to use. I use AutoCAD Civil 3D professionally and this is definately easier than Civ3D - for this purpose. You should have no problem viewing, modifying,disassembling, or redesigning the plan to suit your desires.
I created components that reflected the actual dimensions of standard lumber then used these to virtually build the clubhouse. As a result most of the project went together like a kit. I was able to precut and predrill components on the ground then assemble them in place.
In the design process I got obsessed with the weight of the proposed green roof. I was concerned that the structure would collaps under the weight of wet soil. As it turns out, my concerns were misplaced. I didn't pay enough attention to the foundation and as a result the entire thing has a slight tilt to the right rear corner. The silver lining here is that the roof drain is now at the low point! Standard framing methods and materials will have no trouble with the weight, but it must be properly supported. A good municipal plan examiner would have pointed that out had it gone through a plan review at the city B & I office.