Aside from the above-mentioned writing of vignettes the only time I’ve ever done “solo RP” was… I can’t even remember what game it was, maybe Serenity? It had a space and cargo system, cargo hauling made money, but you only got XP if you were actively posing? I don’t remember the details.
But there I was, elbow deep in a solo flight from one place to another, hauling my mystical space crystals or whatever trying to make an extra spacebuck, posing “Pavel continued to stare blankly out of the main window. Occasionally, he was tempted by a thought, but he was never one to let himself be distracted from his duty.” Etc, etc, etc. I made it a challenge to myself to become as weirdly Douglas Adamsian as I could in my writing style.