It's probably going to be a good few years before the first manned mission to Mars so until then, you'll have to make do with Mars Explorer Widget.
Mars Explorer Widget is a free 3D game which works inside your browser and sees you explore the volcanoes, plateaus and lava flows of Mars with a few stunts thrown in for good measure. You can even play Mars Explorer Widget against friends (or even the 18 year old developer himself) over the internet although playing them at what is not quite clear.
Mars Explorer Widget doesn't really offer any "challenge" as such, although it is fun imagining what Mars might be like to race around. It's probably most suitable for kids or those learning about planets and what to get a general idea of Mars' landscape. As a game though, it really doesn't hold any interest at all. It simply extends itself to a few races and chasing other players around the planet.
Mars Explorer Widget is a fun and educational trip round Mars, although to call it a game is stretching it a bit much.