I've always wanted a space simulator, but other ones seemed to either be completely overwhelming or unrealistic. I found out about KSP, and it's exactly what I wanted.
You design spacecraft, launch them, and try to keep them in orbit. It's sort of a sim, but very accessible to newcomers. Once you figure out the calculations to stay in orbit, of course. It's open, you could go to the Mun by transferring orbits, or you could brute force it and try to intercept it.
There are a bunch of mods too, anyone here play it?