There is plenty of scope to adjust playhouse designs of these to suit your garden and your kids. Once our imaginations conjure a design that suits, the kids imaginations will take over and occupy them for hours on end.
We have also made play equipment for toddlers that can be built to specs for indoor or outdoor use. This one (in production) is called a Pikler Triangle. Find out more about them here.
The one below can be a hideaway for the kids or readily adapted as a hide for bird and/or wildlife watching.
And not forgetting the humble garden log stores, sheds and tool stores!