This box is a GPS puzzle build as a wedding gift by Mikal Hart. It only opens when it's in a specific location, and the puzzle involves discovering just where that location is.
The box has a button and an LCD display on the lid. When you press the button, the display will show you how far, in kilometers, you are from the goal location. It doesn't give you directions, so you need to triangulate where you're supposed to go via trial and error. Oh, and you can only press the button 50 times.
There's a pretty sweet disguised back door built-in as well, just in case the battery dies or the GPS unit fails.
Spoiler: the box only opens when brought to Île-de-Bréhat, France. As for what's inside?
With all the electronics, there wasn't much room inside the box for anything too substantial. I put in a few local (US) gift cards to entice them to visit soon, a set of Kazuo Ishiguro audio books (on a USB key), and an overly sentimental card. And of course, as I pointed out in the card, if either of them fancies doing a little Arduino development, there's a perfectly good Duemilanove to play with, not to mention an LCD, a servo, and a GPS.
I guess the puzzle itself is present enough, really. Pretty cool stuff, no? [Arduiniana via Make]