The standard solo recommendation currently is Ironsworn: great mechanics, fun, and free. It’s easy to play as all the rules can be explained as: throw dice and the result can be good, “good but (something not so good happens too)”, or bad.
You also can get Black Streams Solo Heroes. A small supplement with rules for solo OSR. Maybe Cairn or Mausritter can be easier if you have no experience. Even Mork Borg has a free version, combined with the solo rules Solitary Defilement can be amazing if you like the setting
Some solo games are more focused on the journaling side, pretty much making up what happens based on some random prompts.
Another option is getting a game master emulator. There are lots for free that you could use pretty much with any game. “One page solo engine” is a great option.
I always liked the Monty Python one, and want to try the pirates themed one. Fluxx games are really fun indeed.