"Mars or Bust" from Stanley and His Monster #109 (DC, 1968) by Arnold Drake, Bob Oksner and Tex Blaisdell
First appearing in Fox and the Crow #95, Stanley and His Monster had taken over the book completely by issue 109, and closed the book with 112. I don't know if Bill Watterson ever took inspiration from this strip, because it very much has the same basic premise as Calvin and Hobbes, but regardless, it's nowhere near as funny or charming. Kids' fare, with some amusing art here and there, but little more. If you want to read some S&HM, I'd rather you found Phil Foglio's 90s mini-series, which tied into Gaiman's Sandman, believe it or not, while remaining a cartoony humor comic.