One of the best dollar store purchases I've ever made for my relationship is a dry erase marker. I keep several in the bathroom drawer and use them regularly to scribble little notes to my wife on the bathroom mirror. She often does the same. These are usually just short little love notes, wishing each other a good day or mentioning something we love about the other. We also have a small dry erase board on our fridge that often receives similar treatment, though we add more logistical items there too (went for a walk, dishes are clean, etc). The bathroom mirror though? Only love notes. It takes just a few seconds.