hanging from a tree is a cool idea for a mysterious garden feature.
Keys mentioned in the slideshow at the link.
I see you can buy collections of old keys on Ebay for USD$20...
ADDED: What else would you find in a Mystery Garden?
Obscure, mysterious plants, perhaps? Or maybe you'd have familiar plants loaded with symbolism and meaning, but used in mysterious ways.
I'm sure you'd want to have some odd flowers or unexpected colors. Dragon arum certainly belongs in the mystery garden.
Surely the ideal mystery garden would be designed around an old specimen of some kind, like an oak, or maybe a very old rose.
I think broken statues would be very cool in the Mystery Garden--especially headless or armless statues.
Of course, you'd have a cairn--or an old chimney. Some tombstones off in the corner would be very cool. In fact, old cemeteries (link, link) would be loaded with ideas for the mystery garden.