In a thread like this a long time ago someone recommended Nail’d Steam page

It’s typically 0.99 and just a silly, fun racer. It’s fun to kill some time on.

The killer app for steam deck for me was emulators. I played through some Zelda games (windwaker, OOT via ship of Harkinian). And some Pokémon. What’s nice is for emulators the battery life feels near infinite, and you can hit the power button for instant sleep. Makes picking up games on down time really easy.

Last recommendation is outer wilds. Played great on the deck and the controls are good.