I also have a small, crowded desk, right next to two of the tech work stations and no cubicle walls to block their view of what I'm doing, so I cannot blatantly do my own stuff. I do a lot of clipboard held Sudoku (with official looking paperwork as the top level) and sneak sketches.
I have almost enough clear space on my desk to hold a half sheet of paper. And only because I have my computer pushed back. Half a sheet also easily slips under the keyboard for a quick hide. I have been doing sketching on half sheets of paper to keep myself from getting too bored.
Below are a few examples of these. I started with just random whatever and then went to themes. I tried my hand at practicing drawing characters by the late Luisa Felix to post on her remembrance Facebook page. I worked on designs for a couple of my own characters, Athena, Big Bug and Atlas, and this past week started doing scenes from the Untold Tales of Atlas.