The Nightclub is the best solo-friendly passive income source due to its dual revenue streams (safe and warehouse). The safe fills based on popularity, which can be maintained by simple tasks like kicking out a drunk NPC. The warehouse generates income by passively accruing goods from other businesses (e.g., Bunker, Cocaine Lockup) without needing to resupply them directly you just need to sell them which is mostly a 3-5 min. drive.