Tin foil coworker's workspace.
This prank requires quite a bit of time, so you're going to have to wait until your victim is out sick or on vacation for a few days.
Step 1: Take a photo of your coworker's desk. This photo will be used to make sure you put everything back exactly where it was.
Step 2: Wrap everything you can in tin foil. This means the desk, monitor, chair, phone, etc. The more thorough you are the better the outcome. For instance, don't just wrap the keyboard loosely. Instead, take the time to wrap and press the foil down so that you can still make out the shape of each key.
Result: When your coworker comes in to work they will see their desk all wrapped up in nice shiny tin foil. The wow factor is pretty big on this one.