Simply take on old picture frame with a glass cover and put it on your desk and use a marker to write on the glass. The ink dries in a few seconds yet wipes of easily with nothing but water. You can pretty much use any marker, even permanent marker though they tend to need a little extra rubbing to get off.
For a fun extra, add a pattern to where the picture would go. Obviously, the paper should be a fairly light color so that the ink will be visible against it. I recommend using old wrapping paper for something pretty, or simple lined paper for practicality. If you’re feeling especially crafty, you can use stamps, ribbons, and other such materials to make a festive background.
Also, a decorating tip would be to get a couple frames that match and hang them on a wall together. Each one can be for a different topic such as “To Do,” “Groceries,” “Don’t Forget” and “Emergency Numbers.” For students, each frame can hold reminders for each class or subject you’re taking and hung above your desk. Be sure to have a variety of color markers to keep things interesting.