I've thought of a new way to not work without looking like you're slacking! I've actually been instinctively doing this for a while and just realized it. Eating. It's brilliant. If you've got nothing to do and you're slacking off, when someone is about to walk behind you and you have to go to your emergency "could be legitimate work" screen (you know, the spreadsheet, the Word document, whatever else it is that you do), instead of staring blank-eyed at the screen until they walk away, try a new approach: lean back casually and take a bite of your banana, a sip of your tea, or start snacking on chips.
Think about it! It's not slacking at all to take a moment's break from your work to nurse your hot tea in the morning or have a quick snack. You can lean back and enjoy your life, if only for a moment with this subversive act of resistance! Plus, it's way more interesting than pretending to figure out the numbers on a spreadsheet. It's also tastier.