I used to take breaks from academic work by playing Starcraft II, but I haven’t loaded that game once since I got the demo and eventually the full version of Kerbal Space Program (KSP). It’s unambiguously one of my favourite games of all time. Nonetheless, there are several steep learning curves. Each stage of your development in KSP roughly approximates an area of knowledge necessary for real rocket science: maneuvering, orbital mechanics, rocket design, the understanding that a lot of your astronauts will die because of your mistakes (especially if you tinker with spaceplanes).
The full version of the game doesn’t come with a craft that has a decent chance of landing on the Mun and returning safely to Kerbin. I have modified the Kerbal X to be able to do this with amateur piloting skills.
This craft relies on a couple of plugins: MechJeb (fly to the Mun, land, and return home without using it at all for major geek points) and Kerbal Joint Reinforcement (your rocket may shake apart and explode without it).
Here it is: Modified Kerbal X with more fuel and engines and extras to help you land on the Mun (version IV)