Rust is pretty fun/terrifying/allows for fight or flight scenarios which will happen a lot. It's probably the most complete non-zombie survival fps game I've played and is only getting better over time. I think it's $20 for early access as it is still in alpha on Steam, but the most recent update has fixed the majority of lag problems. I have a nice PC, but it's far from the best out there and I am still able to run Rust on the highest graphics settings @ ~40fps.
I wasn't sure if I'd like the post apocalyptic aspect to it but it feels like an island where the government has marooned a bunch of savage people, wiped their memories and you have to claw your way to the top of the food chain by gathering resources, crafting tools/supplies and building up your own base or raiding others, starting by waking up naked with only a rock and a torch. There's big rad towns which house a number of items you can unlock with their blueprints and the dev team is currently working on somewhat of an RPG aspect by adding a leveling tree to the things you can unlock.
Words I can say cannot begin to describe how fun and addictive this game is and I wholeheartedly recommend it. 8/10.