Kerbal Space Program is definitely more of a sandbox than a game. A sandbox in which the “sand” is parts of spaceships and the “box” is a solar system. Using a realistic — though simplified — simulation of space flight you hurl your creations to the sky, where they may soar gloriously to their destination… or not.
Few games, if any, leave me as satisfied or with the same sense of accomplishment as Kerbal Space Program does. Maybe it’s the real physics the game uses, or the real(ish) scale of the game, but achieving something for the first time feels better here than it does in any other game I’ve played, whether it’s just getting out of the atmosphere, reaching a stable orbit, or landing on another planet.
The Kerbals themselves are a little wacky and somewhat slapstick, which adds to the charm as far as I’m concerned. The idea of these little creatures exploring space just for the sheer unadulterated fun of it in varying states of exhilaration and terror is infectious. The satisfaction of landing them safely on Mun is fantastic, only matched by the utter crushing disappointment of failing and smearing your (not so carefully) constructed vessel across the landscape.
As an added bonus you learn real (if somewhat entry level) orbital mechanics and rocket science here. Or at least you do if you manage to get anywhere in the game. If you play it expect to become familiar with the Oberth effect, transfer windows, how to perform an orbital rendezvous (hint: it’s not “point toward it and turn on the engine”), and the tyranny of the rocket equation. (To those in my D&D groups, it’s like “horse math”, only worse.)
On top of all that, it’s not just space you can explore, if you want to take a break from that. You also have the parts needed to construct aircraft and very rudimentary boats, if that’s your thing.
Personally I’ve gotten a huge amount of enjoyment from this game, and would recommend it strongly to anyone with an interest in space travel. Seriously, check this one out.