Monster Hunter Rise is great on handhelds like the Deck.

Dragon Quest XI is one of the best JRPGs I ever played, and it has an extensive demo (about 10-15 hours) that lets you transfer your save game to the full game.
Problem is the save transfer didn’t work on Linux last time I checked about 2 years ago. Maybe Valve fixed this since. Otherwise if you have a Windows PC you could transfer the save game there, but keep in mind you won’t be able to download the demo anymore once you’ve purchased the game.

Welcome to the joys of Steam OS!

Get Rimworld, and write a letter to your friends and family that you wouldn’t see them in a while. Consider spending the rest of the money on IV stand so you stay hydrated.
Rimworld is a drug, it should be classified as a controlled substance.

I hate using a joystick for camera control, so I only play 2d titles on my deck. Mostly DRG: Survivor, Tribal Hunter (no, really. It is excellent), and Hades.

BG3 is another good bet too.

Dome Keeper

Be sure to check out Steam’s own “great on deck” category in the store. Each game will also have an indication from Valve on the quality/playability with regard to the Steam Deck

There’s already a very long list of excellent suggestions , so I’m going to chip in and add only two others. Tunic and Tinykin. Both are Deck verified and on sale right now.

I also was gifted a Humble Bundle game for Batman Arkham Knight.

Then perhaps redeem the game and complete the bundle at https://store.steampowered.com/sub/320795/ to play Arkham Asylum first?

I’ve had great fun with Jedi Fallen Order and Hollow Knight / Silksong

I’ve put a disgusting amount of hours into Emberward. It’s Tower Defense but no mtx and has a really fun Tetris blocks gimmck to build lanes.

https://store.steampowered.com/app/2459550/Emberward/

Yes it’s early access but it doesn’t matter, the game is nearly done anyways, also on sale currently. There’s good Steam Input profiles for the Deck. Dev is adding controller suppport for 1.0 but this doesn’t really matter on a Steam Deck.

I’ve been replaying Dragon Age Origins at work modded via Nexus Mods and running DAO-Modmanager via WINE under Steam’s compatdata prefix.

Balatro and Slay the Spire both play great on the Deck and have amazing replayability.

Megabonk, Balatro, or Cyberpunk are my rotation. Hunt those steam sales

You’re getting all kinds of recommendations for Steam games. But what about free games? In desktop mode, the “discovery” app let’s you install programs, including games.

There’s tons of free games. I’ll admit a lot of them aren’t the highest quality. They are mostly open source clones of more popular games.

One game I recommend in particular is Super Tux Kart. (Tux is the mascot of Linux, and SteamOS is Linux based.) It’s a kart racer game. It’s one of the highest quality open source clones out there, though. Great graphics too.

If you own Minecraft, the Prism launcher will allow you to run it in the Deck. Just go into the steam menu in desktop mode and click “add non stream game” to add any apps you install this way to your steam menu. Now they can be launched in handheld mode as well!

Not sure what games you like, but I will suggest Terraria to almost anyone. 50% of currently on steam. https://store.steampowered.com/app/105600/Terraria/

Satisfactory sucked up about 3 months of my time and thinking about it as I type this I really want to start a new play through.