Super Wolfenstein HD is quite the thing. It's Id's classic FPS, reimagined with physics. And silliness. You can download and play it for free.
It was created for the Indies VS PewDiePie jam—the same competition that gave us the relaxing Where Is My Hammer? Here there's more challenge, thanks to enemies that stumble and ragdoll around. Ammo can be sparse, too, which can leave you fumbling around with a shovel as a dog vaguely ambles in your direction.
Really, it's all an amusing and ridiculous distraction, as I can prove via the following