A while back I linked to a hidden door picture gallery and mentioned
When my dad remodeled our attic into a bedroom (for me) he put up some bookcases and mounted one on hinges, so you could access the crawlspace.
Having a bedroom in the attic was good for freaking out some of the freshman girls in art class when I was a senior in high school — When a Stranger Calls (“have you checked on the children/the call is coming from inside your house!”) had come out over the summer, so mwuhahahaha, but the real fun was when I was younger, because of the hidden door and the crawl spaces.
Here’s the handle for opening, so it’s not super-secret, but I have no complaints. It wasn’t meant to be hidden from The Hardy Boys (or Nancy Drew). It was meant to be easily opened by kids, so mission accomplished.
The play area behind the bookcase was comfortable for a couple or three ~10-12 year-olds, but the crawl space runs the length of the house, and there was a smaller version on the other side, where the wall connects to the ceiling at a lower point. Add in a chair and a blanket, and it made for a great tunnel system for playing The Great Escape (or Hogan’s Heroes, or Stalag 17, depending on the mirth quotient you wanted to assign the bad guys. Hey, we were 10-ish.)
(The vases on the upper right, on the regular shelf, were his work as well; he did a lot of wood-turning/woodworking projects.)