I’m not sure if this is the best recommendation looking at your criteria, but Slime Rancher (1 and 2) are chilled games that have beautiful and colourful visuals. To me it feels like a summer vacation - one where you’re staying at a farm and catching adorable slimes.

Another recommendation I’d have is Tearaway unfolded. It’s a super charming adventure in a papercraft work. It really stuck with me and felt like an adventure. It’s only on PS4/5 as far as I’m aware though, but if you happen to have one of those I guess you could cast it to your Steam Deck using Chiaki. Or on the off-chance that you have a PS Vita, the original version of the game was on there.