We'd unpacked Cairn with my Sis by playing 1-on-1 one-shot that spanned over two brief sessions. Cairn is an overly popular rules-ligt OSR ttRPG by Yochai Gal. The engine Player's and Warden's books are freely available together with some short intro scenarios, and a whole collection of 3rd party original adventures and conversions.

^ our minimalistic PlanarAlly board for the Temple of the Moon Priests one-shot adaptation

The system has some similarity with Electric Bastionland (attacks always hit), but also with Traveller: stat pools are the hitpoints, namely STR for health, DEX for agility and WIL for the mind. Accumulated fatigue eats through the character's inventory, so does the spellcasting.

Essential rules are so compact that they can be explained over bare minutes before the actual play. Character creation is also blazing fast, but I like the variation of the playable archetypes and different approaches to their characteristic descriptions! The books also contain useful tools for open environment and dungeon-crawl. Moreover, there's Kettlewright - an official online generators collection for Cairn. With it we've made a few characters in mere seconds, selected the two most interesting of them and were ready to descend!

^ the Thief and the Illusionist that teamed up to explore the temple.

One more resemblance between Traveller and Cairn: no character progression apart from what loot they earn and how far in the story they progress. That and light rule set in general favors one-shots and short scenarios. It's really great for fast preparation and play - even Whitehack would take more time to explain, not mentioning the level of abstract thinking and improvisation required to play it (and the absence of official online tools).