I should preface this by saying I don’t actually have a steam deck yet, so I haven’t tested these on there. So I’m only commenting on the games themselves. These are listed as deck “verified” in the steam store, though.

One I haven’t seen mentioned yet is Yoku’s Island Express. Breezy summer vibes, not much difficulty. It’s kind of a pinball metroidvania.

Tinykin is another game with a very cozy/low stakes feel. It’s an exploration/collectathon platformer with cute environments made up of household objects.

Littlewood is a life sim sort of game, kind of like Stardew Valley, but it’s extremely chill. There’s no time limit or anything like that.

And others have mentioned these, but Toem, Alba, and Donut County are all very good and gentle games too.

Oh, and Tchia. That one has some dark moments at times, mostly in cutscenes, but when you’re actually playing it’s mostly gentle and island-y.

Maybe also Wuppo? It’s a strange one. The story and humor and animation are pretty great in that one, but there are some boss fights that can get a little frustrating. It’s mostly a fairly chill platformer, but then it’s got kind of bullet-hell-adjacent bosses. I still really like the game, but it’s not quite as purely relaxing as some of the others here.

Pikuniku is kind of in the same position as Wuppo, but I liked it a bit less. The humor feels a little more forced or stilted, and the frustrating bits are because the controls are kinda floaty. My niece really liked it when she was 8, though, so it had that going for it.

Hope this helps! I’ve been looking for this kind of game a lot the past few years