Baba is You - A puzzle game that's so simple they even write out the rules in big letters on every single level for you. Easy, right? Right?
Celeste - A platformer with 8-way airdashes. I'm sure you've already heard enough about how amazing the base game is, so I just want to take a moment to shoutout the incredible modding scene for more content. My favorites are Spring 2020 Collab, Strawberry Jam, Glyph, Conqueror's Peak, and Into the Jungle.
CrossCode - Phenomenal action RPG. Combat is fast and explosive, dungeons are very obviously Zelda inspired but with way more puzzles. Packed with tons and tons and tons of sidequests, many of which put unique twists on the combat system to keep you on your toes. Make sure to grab the epilogue DLC.
Crypt of the Necrodancer - Rhythm game/roguelike sounds like the strangest mashup ever. But what's even stranger is just how well it works. It's just a matter of keeping 4/4 time, but forcing a steady pace forces you to think fast and not make any mistakes. Every death is clearly your fault as every enemy is designed to be beatable using only a base dagger without getting hit (and indeed there's a challenge character that forces exactly this), but dealing with swarms is where it gets complicated. I especially recommend trying to speedrun, playing for speed really makes this game adrenaline-fueled as you have to pace yourself judging how much time you can afford to gather items if you want to make sub-15 or sub-10. Danny Baranowsky's soundtrack absolutely delivers. Get both the Amplified and Synchrony DLCs.
Dicey Dungeons - A roguelike, uh, dicebuilder? I'd call it a sort of Slay the Spire-lite, not quite as much depth and you'll probably fall back on the same builds a lot, but it's still good casual fun to unwind with.
OneShot - Fairly reminiscent of Undertale, if you liked that you'll probably enjoy this too. And like Undertale I don't want to say too much, take my word for it and let it surprise you.
Panel Attack - FOSS clone of Panel de Pon/Puzzle League with modding support and netplay. Also check out Super Plexis for a commercial clone that's still in early alpha but has a lot of ambitious features planned.
Simon Tatham's Puzzles - Big FOSS collection of logic puzzles. Most of them have lots of parameters you can play with to generate any size board you want at whatever difficulty and various other tweaks.
Skullgirls - Still the best damn fighting game of all time. Almost never not on sale, hell you probably already own this by now and may not even realize it.