Calvin and Hobbes went to Mars in a story arc that began on September 12, 1988 and ended on October 1 of the same year. The arc only included regular strips; the colored picture below comes from the cover of Weirdos From Another Planet.
History
Prelude
Calvin is exasperated by the extent to which mankind has polluted the Earth. Refusing to inherit a planet so extensively profaned, he looks for a new place to call home. Hobbes suggests Mars, and Calvin gets him to pack for the Red Planet. He sets up a ramp for his wagon to lift off from, and launches himself into space with Hobbes on his wagon.
On Mars
The martian
The martian landscape
The wagon lands rather smoothly Mars after no more than a few hours. Calvin and Hobbes look for a place to set up camp, and come across NASA's Viking probe. Calvin unpacks, but before he and Hobbes can turn in, though, a moving rock catches their eye and the two come across a Martian, who is terrified of them. They run off, and the alien's reaction gets them to think that they may be trespassing somewhat on the planet. Guilty due to their intrusion, they decide to launch the wagon and head back to Earth.