Awww.... you guys are hauling out the nice ones.

My mom's neighbor has just such an affair he made as a playhouse for his kids. He kept it in the front yard. NOT pretty... but I'd imagine it was a heck of a lot of fun for the kids!
Septic tanks:
Some are fiberglass... There are some water storage tanks which would work too. Kinda hard to fit windows onto though.

mkw