This is a great idea. A Mug with a chalkboard surface on the outside so you can write any message you want and just wipe it clean for the next one.
Why would you want to write a message on your coffee cup? Write your name so people don’t accidentally drink from it, leave a message for someone in the morning, specify how much sugar and milk you want in your coffee, keep a record of how many cups you drink. The possibilities are endless.
You can buy it here for about $8.00 USD + shipping. But you could also make your own for far less. Pick one of your cups, go buy some chalkboard paint and apply a few coats to the outside of the cup. Arieanna and I do this to label the contents of the jars in our pantry (ok, it’s just a shelf…) and it works great.