I find the steam deck (albeit with a tiny amount of Linux tinkering skills) to be amazing at playing really old games. For example I got Need for Speed Underground 2 working on it which is max nostalgia for me personally. I used to play a lot of FPSs too, quake3 era and later, and basically all of those work great too, though not something that is particularly good on a controller. The author might find that getting the nostalgic titles working for them is rewarding as well.

Check out chop goblins. Made by the same dev as Dusk and Iron Lung (both highly recommended if you like old shooters and horror games). It's cheap and designed to be finished in less than an hour, with the idea that you'll play it once, put it down, then play it again a week later.

I'd argue that games like WarioWare and Animal Crossing are also pick-up-and-play games, though admittedly they're locked to the Nintendo ecosystem (though I've heard Yuzu runs wonderfully on the Deck).

Namco also just released a remaster of We <3 Katamari a few months ago, which, combined with Katamari Damacy Reroll (remaster of the original game), makes for two more pick-up-and-play games (they both have an overarching story that ties the levels together, but each level can be completed on its own and have a lot of replayability)

I think Voices of the Void (not on steam, it's on itch.io), while not a typical pick-up-and-play game, can be played as one as you could play for an in-game day or two and then put it down. Same with The Long Drive, which, imo, is the best driving game (not racing or car game, driving game) out there right now, especially if you mod it (you have to join the dev's discord server for mods though). You could drive for a ways and then save and quit when you get bored.

Do you want short horror games that can be completed in an hour or two? There are a ton of those out there. Iron Lung is one of them, but there are a lot of others like Squirrel Stapler (same dev again), or games made by Chilla's Arts, or Puppet Combo (turn your sound down though, Puppet Combo's games emulate low-budget slasher fics and they LOVE using cheap, 2 billion decibel jumpscares). Wanna see what indie devs are making in regards to horror? Check out the Dread X collections. They're collections of 10 different horror games and demos, each one typically short enough to be completed in an hour or two play session.

I'm sure there are many more out there, it's just that it's become more difficult to find them because of how many games there are now.