Free plans to build your own indoor playhouse! Sized similar to play tents, so it fits anywhere, but won't break and feels more like a playhouse.  This well thought out plan maximizes material use.  About $50-$100 to build (depending on local lumber prices).  

UPDATE: After many years of use, kids climbing on the roof, "decorating" with crayons, the playhouse is still in good condition and used all the time!  This is a great plan that my kids have loved for

Build your own custom sized bookshelf! This plan includes a custom designer so you can build your bookshelf in any size.  Free plans with step by step diagrams, video tutorial and more from Ana-White.com