If you like open source gaming, Cataclysm: Dark Days Ahead.

Turn based zombie (and other creatures) survival game with very intricate crafting mechanics. It plays in the same style as a classic roguelike (think stuff like Pokemon Mystery Dungeon) when it comes to movement.

Anything in the game can be broken down into components, and most things can be built back up from those components. The vehicle building system is also very solid, you can make a fully customized hybrid car with remote turrets, a horse-pulled shopping cart, or anything in-between.

The game has a pretty big focus on realism, there's concepts such as sound and smell which determine how you are detected by creatures. I remember reading about a user on the community discord server who researched the properties of bug chitin because they wanted to change the giant bugs you can fight in the game.

The biggest downside is that it's pretty obtuse to get into if you have no experience with similar games. Movement is most efficiently done using the numpad, as the 1,3,7,9 keys work as diagonals. Most keys on your keyboard perform a separate action (e.g. 'e' to examine, 't' to throw, uppercase 'E' to eat etc.). You'll find yourself checking the controls often. Similarly, there's basically no tutorial. Or more specifically, there's no way to make a good tutorial for how much you can do in the game. The mechanics are all so diverse and so deep that the only realistic way to learn about them is either trial and error, or asking questions on the community discord server.

That being said, it's one of the best sandbox games out there. And it's free, so why not try it?