I used to tuck a leather notebook under my arm as I goofed off, er, I mean, went about my business. It lent me great dignity. And then in the last few years before I retired, when cell phones were becoming ubiquitous, if I saw a 'suit' nearby, I would lean against a wall with a cell phone at my ear, talking earnestly. If said suit caught my eye I would give him a solemn nod as he passed.
This works well especially if you are carrying papers as well. Another trick for programmers with multiple monitors is to keep a really complex piece of code that no one will understand up on your monitor that is visible from outside your cube, and then do whatever you want on the monitor that is facing the cube wall. Of course you will have to have a shortcut mapped (boss key) that will minimize whatever you are doing just incase someone unexpected pops in, and a rearview monitor mirror helps too.