You might have seen our post on this last week:
It's a working reproduction of the Mars Curiosity Rover made with LEGOs, but it's a bit complicated for anyone without a decent amount of robotics knowledge. Plus, you need to have some Mindstorms NXT and TECHNIC parts lying around. If you want something a little simpler than programming a working LEGO rover, you could always build a scaled model of the MSL out of regular LEGOs.
Not only is this easy to build and looks great, but the step-by-step instructions also give details on the actual parts of the rover that you're replicating. Playing with LEGO and learning some science? That's a win-win if I ever saw one, especially since this LEGO Curiosity was built by Stephen Pakbaz, a mechanical engineer at NASA's Jet Propulsion Laboratory who actually helped design the real Curiosity rover. Maybe one day he'll show us how he built his Sky Crane, too:
For the Curiosity build, you're on your own for tracking down the individual LEGO bricks, but there's really nothing used that wouldn't be easy to pull out of a spaceship set or ordered online.