Tabletop Gaming

There's a ton of good free RPGs. The most complex and complete one that's also made for solo play might be Ironsworn, but I'm personally a fan of Mouseritter for the unique setting and streamlined mechanics.

You can find a lot more on Itch, which is pretty much the place to go for downloading RPGs. Besides the free ones, you can also get use the 'community copies' feature to get some of the paid ones for free if you can't afford them. Artefact and Thousand Year Old Vampire are two solo classics you could probably get that way.

I looove Mausritter. It comes with a simple and beautiful setting and has everything you could want from a simple rpg.

I've been playing a fair amount of Heroic Tales lately, and it has a Solo section and is available for free. It's a D6 dice pool system with two "kinds" of dice. You can have 1, 2, or 3 Check Dice (success is if any of them roll a 5 or a 6) then your character can get up to 6 Skill Dice (success if it rolls a 6) for a roll based on stats or gear. The difficulty of what you're attempting determines how many total successes you need from a roll. It also has a failing forward option where there can be partial or mixed successes, where you get some or all of what you wanted, but something goes wrong as well.

I've also heard a lot of people grab Iron Sworn and use the solo tables as the oracle to run any system in general solo!

Not sure about solo games, but check out ten candles for a game where you won't need a lot of prep.

If you like stuff like D&D, you could try to check out ways to simplify character sheets (or pre generate characters) and use existing modules for one shots.

There's a lot of free ones you could start with - I kept getting recommended a oneshot called A Most Potent Brew when I was looking for one. It's a pretty fun time and you won't have to come up with something new to run it.

If you want to further simplify gameplay for D&D, you can have a coin toss determine if you hit or not, and a d6 (regular 6 sided die) for damage. Movement is 6 squares a turn. Play it more like a video game, just on paper.

A friend of mine introduced me to Honey Heist a while back. It’s incredibly easy to play and very fun.

Basically, the party is a gang of bears trying to pull off a heist for honey. The game cascades into success or failure depending on rolls and the players progressively turn more bear-like or more human-like as the one-shot progresses.

Doesn’t take a lot of skill or planning (and it’s more fun in this case to fly by the seat of your bear pants). I’d recommend a minimum of 3 people (1 GM and 2 players) but more people=more chaos/fun.

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'm not sure what the appeal of solo play is, because roleplaying is all about building collaborative stories. If you're doing it solo, you might as well just write a novel (also very cool). I guess you could use some kind of random tables or something to help you come up with material when you're stuck with writer's block, but that's about all I can see it being useful for.

Anyway, a fun, simple, and free (at least in the sense that you can acquire it without paying money, not in the FOSS-type sense that you are free to modify and distribute it legally yourself) little system is Risus.