Not a complete list, but may help to provide a starting point.
Individualize and personalize, add a signature of original ownership.
Dates become milestones in life. A landmark marking a specific date in history. These can be birthdays, anniversaries, or another specific date which can display month, day and/or just a year.
One, two, or just three words can be very powerful.
Can come from a favorite book, song, or movie said by a character, actor or singer. They can derive from a close friend, or a family member, or possibly overheard by an unknown person. These usually activate an emotional pulse in our hearts and minds. The right quote can be helpful during times of need that may offer hope and courage in the future.
A small simple communication that can invoke love, ramble of nonsense, or be a holiday greeting. A personal detail of words between friends or more than... An intimate note or a reminder of something important.