Hardspace: Shipbreaker
This one is probably only really zen if you play with endless shifts and infinite air on, but if it is, there’s a lot of zen to be had in dismantling a spaceship in zero G~

Ship Graveyard Simulator (And SGS2)
Most like Hardspace, except you’re on the ground and having to do quite a bit more in terms of the actual dismantling, but you also basically can’t die, and if you do manage to have something happen to you via explosive or radiation (in the SGS2 DLC) you just come back a little bit away.

Power Wash Simulator
According to some, this also works as a diuretic, so you may want to watch out for that. The ultimate in this kind of zen game, there’s no fail state at all, just you, a stream of water, and a world of immense filth and minimal logic.

Please feel free to add your own~ these are just the ones I’ve played somewhat recently that one can play on a kind of autopilot while actually concentrating on something else. :)

Yeah, I was about to say, I wouldn't consider this a particularly zen game if you're trying to push score or speed with high tier ships in standard settings. :)