hawkeyefan
Legend
We've spent almost every Sunday of the pandemic playing casual and indie RPGs requiring little to no prep. A huge source of these has been Grant Howitt's one-page RPGs, such as Sexy Battle Wizards. While I'm not normally a huge PbtA fan, we've also had fantastic fun with games like Brindlewood Bay and Pasion de las Pasiones. I've run lots of GUMSHOE (including Swords of the Serpentine), I've playtested a new low-prep game I'm working on called LOOT THE KINGDOM, and we haven't even come close to running out of games we want to play.
I just wanted to cosign the above. Grant Howitt has made a few dozen one-page RPGs that are all freely available. There may be a couple that seem more of a novelty than something you would actually play, but the majority would make for fun one offs that are simple to learn and run and play.
I also think that the games he's written with Chris Taylor, Spire and Heart, are both relatively simple to play and to run, although the learning process of some elements is more involved. Compared to most they're relatively simple, and have a core mechanic that is easily grasped, but they do have a lot of player options and the like which can seem overwhelming to some folks new to RPGs.