Hi from Space ;-),
there is a truly well made space flight simulator called Orbiter:
http://orbit.medphys.ucl.ac.uk/ . It is closed source, but you can add spacecraft models and such. It also has a quite large community. I played around with it alot a few years ago and it is truly great! Just one thing: It simulates spaceflight as realistically as it comes. Therefore it will be of great help, if not a requirement, if you know a thing or two about orbit dynamics. You can fly to the Moon and Mars, whatever you like. But the transfer to Mars will take the usual 8 months (you accelerate the simulation, though) and you just missed the launch window a few weeks ago...
In principle, JSBSim could do the same (if anyone from JSBSim is here: Hint, hint...). It might need to be pimped with more complex gravity models including Moon, Sun, Jupiter, etc.
Which brings me to another idea: There are terrain models plus imagery of the Moon and Mars - how about extending Outerra beyond "Terra"? The Moon should be simple - no atmosphere ;-).