Some things I like to do on a regular basis:
1. Yearly, I make thank you cards and send them to people during the holiday season. This is a result of annual close calls. I wrote a blog post on it here.
Year 1, 2014: Santa on a Velociraptor
Year 2, 2015: Day and Night
There are messages written with UV-activated and phosphorescent inks.
When you place it under the sun, the UV-activated ink will turn purple. In this card, some of the letters are in plain watercolor so you can decipher the message after you give it a solar bath. The yellow letters glow in the dark.