I know this is very old, and the birthday this was meant for long gone, but I think it would be amazing to have a ting like this with a link to a geocache hunt on it.
www.geocaching.com/ - It could lead to the container being in a special place for the night (or clues to get to it) and filled with things that are favorites or bring up memories. It could be private until after the night. Then, the couple could leave some of the items for the next people and photos or links to photos from the night or their life. They could encourage other people finding the geocache box to do the same, and come back every year to see what the year of shared memories has taken from and brought to the box.
Heather