I had a lot of fun checking these out today!

I’m not sure if PICO-8 games would fit with this list, but there are a lot of fun quick ones such as:

Snowfight: https://www.lexaloffle.com/bbs/?pid=mb_advent2019

Sonic16x16: https://www.lexaloffle.com/bbs/?pid=97365

U Dead: https://eggnog.itch.io/u-dead

And several others (I have a curated list here[1]).

They work on mobile and desktop, don’t require registration, etc. But maybe you wouldn’t want the list to get overwhelmed with PICO-8 games.

[1] https://memalign.github.io/p/pico8.html