There was a long period of time where KSP 1 didn't have a career mode, so people were always coming up with things to do on the forums and whatnot, ranging from single missions to entire playthroughs. Not sure how easy it would be to find those sorts of posts nowadays though. Googling for them will probably just give you a ton of results relating to the modern career mode.

Back then, I liked to recreate IRL space programs (even ones that never ended up happening, like the Constellation program). Recreating real rocket designs in-game has always been one of my favourite things to do.

Another option is to set an arbitrary goal, such as "Plant a flag on every planet and moon." Though, for the time being, that is a hell of a lot easier said than done.