This is an incredibly sweet idea. "Open when you need to know how much I love you; open when you don't feel beautiful; open when you need a laugh; open when you miss me; open when you're mad at me; open when you need a date night, etc.
Remember: Doing Nice Things for your family is just as good as for strangers. Don't tell them about it- just leave it for them to find. A sweet loving message or a thank you or an encouragement. Any kind word is good. The Art of the Surprise Note .