I think all kids like making dens and caves, and ours are no exception. Leo especially is constantly building things (barns, pens, houses, etc. in and out of the house). Maybe because we have been building the house since (and before) the day he was born.
So, this year we decided to make him his own little house. It's made of plywood, cost a little under $200 and took three days to build and paint.
It has a downstairs bedroom with shelves for toys, and an office and playroom upstairs. The kids LOVE it. They play, read books, do homework, sleep, everything in there. There seems to be only one downside as far as Leo is concerned - "How come it doesn't have a toilet?". Well, nothing's perfect!
We'll be doing a full how-to after Christmas. In the meantime, hope you all have a wonderful holiday season.