Chalkboards are extremely trendy right now! To give your guests something sweet to read, display a relationship timeline chalkboard. If you want to make your own chalkboard, you can purchase chalkboard paint at a majority of craft stores.
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.