Hmmmm... Not sure what I would put in a box that I would save away like that. Most of the things that make me happy are things that I keep close all of the time, like the beautiful Tiffany key from my hubby. Photos of "happy" places are my computer screensaver, music that makes me feel good is also on my computer.
Mementos of family members that are no longer with me just make me cry - not exactly a happy thing - and I was so fat in my wedding photos that they make me cringe. I just don't keep much out of pure sentimentality.
All I can think of is a bar of 90% cocoa chocolate and maybe my well-worn copy of Gone With The Wind (which is pure escapism rather than "happy" but...) And I can't very well stick my hubby in a box!