The Pico-8 version of Celeste inside of Celeste

In smash bros, turn the launch multiplier all the way up and turn on sudden death mode so everyone starts at 300%.

Go to one of a few levels (the underground area of Hyrule Temple works, the underground area of Skyworld is better, but it’s best if you make a custom stage)

Getting hit once will send you bouncing around the screen! You only ever die if you get unlucky. It’s hilarious, and we call it “Pinball Mode”. I’ve made a couple custom stages to improve on the experience.

From the title I was thinking of, like, Grognak's Adventure inside of Fallout 4.

But from the body text... Yeah, I don't really do that. I am usually pretty annoyed by how many servers for games I want to play aren't actually playing the game itself, but their own game inside the game and making up their own rules. Which is totally fine, but it should advertise it somehow or not even be public as many of these groups just kick randos out anyway.

I do like a lot of mods that were inspired by these games within games tho. ARMA Life only exists because people were already trying to play the normal game that way, and with the mod the rules can be more easily enforced through scripts instead of requiring hands-on management from an admin.

I played so much pong in commander keen 4.

The real game was too hard!

The first example I ever have seen and my favorite was that Day of the Tentacle had a computer that would allow you to play the entire first game, Maniac Mansion.

My favorite was “Halloween” in Halo 2 multiplayer, kind of a knock off “hide-n-seek” but with a twist.

No shields, no radar, the only weapons were a sword and a pistol, and you only had one life. One player was “Michael Meyers”, that player had the sword and chased down everyone else. All the other players hid while Michael picked you off one by one. Once one player was left, that person became Jamie Lee Curtis and could try to take down Michael Meyers. (To be fair that person should be Dr Loomis or Laurie Strode but it was more fun to use the actor’s name than a character.)

The twist was the music. In Halo 2, you could hear enemy team chatter through the tv speakers and it got louder as you got closer to the enemy. Using that, Michael Meyers would have the Halloween theme song playing so you knew when he was close but you didn’t know where he was. It was truly thrilling, you hear the music, you know he’s close, but not where, the music would get louder and louder as he approached….and then he would walk right by your hiding spot. Or, you’re walking around as the music builds so you know he’s close, you think you’re safe because you have an idea where he is, and suddenly he’s right behind you with a sword in your back.

I play beam.ng and try not to crash.

In GTA 5, get on a train, get a wanted level, try to keep the wanted level for an entire round-trip around the map, then try to escape the wanted level, all while staying on the train and surviving.

(it's been a few years since I tried this and don't remember if those were the exact rules, but they were something like this)

We used to play a "serial killer" game in Red Dead Redemption. The goal is to kidnap a woman without getting seen by anyone. Its actually a lot harder than you would think because female NPCs are RARELY unescorted in the game. It opens up an element of planning and stealth that you (or at least I) don't usually use in that game.

I invented a game called Horse Toss on Minecraft. I don't know if you can still play it, but it used to be the fishing rod pulled exponentially based on the distance, so at like 60 blocks above the mob you hook, the mob would fly about 90 blocks into the air. From there, knock back would throw mobs at an angle depending on where you were when you hit them. If you're below them, they fly in an arch.

You go up on a tall platform with a fishing rod enchanted with knockback 5, pay a diamond and it would dispense 8 horses in a pool below you. You hook the horse, yank it into the sky and try to wack it as it comes down. The pool catches it if you miss so you always have 8 tries. If you hit the horse, it lands in an area in the distance with pressure plates that dispense valuables for for score. The horse dies on impact 99% of the time but of it doesn't it can wonder around and get you a bigger score. At the end, you trade the rod in to get your loot and you can keep the horses if any survive.