My SO and I have been having a lot of fun playing co-op games on the Steam Deck connected to the TV. We recently finished Split Fiction and I'm looking for the next cool experience to try out. We enjoy casual co-op games, nothing too hard or violent. EDIT: pixel art is apparently a big turnoff for her so that's out as well.
Games We Enjoyed:
Split Fiction
It takes two
Untitled Goose Game
Games that flopped:
Stardew Valley - She is open to "farming" games but doesn't enjoy the energy management aspect of the game. It also gave her motion sickness, somehow
Overcooked / Moving out - We enjoy these games but as they start to get more chaotic and hard, the enjoyment goes way down
Ibb & Obb - A bit dull and samey
Portal - Gives me serious motion sickness, completely unplayable
Lego Lord of the Rings - I'm a lord of the rings fan but I hated the gameplay for this game, I found it to be so boring. We quit after an hour or so.
Any of the Warriors games (just don't buy it, don't reward them for releasing the same damn game 20+ times)
Legend of Mana (highly recommended, please don't sleep on this)
Destroy All Humans 2
Ratchet: Deadlocked
Divinity: Original Sin
Resident Evil 6
Halo: Master Chief Collection
Lego Star Wars
The Tales games are long and go up to 4 players. I'd recommend them if you have more people to join.
Be prepared for a few roadbumps while you acclimate yourself to using emulators. It won't be smooth at first, but it gets easier the more familiar you become with it.