A lot of games let you fly planes, but when was the last time one let you fly a rocket? While if that has been what you've been looking for in your time-wasting pursuits, wait no longer, for Kerbal Space Program lets you do just that. As the director/god of the aptly named Kerbal Space Program, you have the ability to launch probes, satellites, landers, space planes, and a whole plethora of fancy little stuff. But behind all of this glamour comes (simplified) rocket science. Much like real rocket scientists, you have to design a craft with fuel and power constraints in mind. Going to the Mun may not be as trivial a task as some may hope.
If you prefer fast paced action, this game probably isn't for you. But if you're willing to think a bit, ponder questions about choosing an engine with a larger thrust to weight ratio versus one with a higher specific impulse, or how you should stage your creation to successfully land on Minmus and return home safely, this might just be your cup of tea. I recommend checking it out.