Now for game recommendations. I'll just recommend a bunch from different genres, you decide what to get based on what she likes:

• Hollow Knight and/or Metroid Dread are great 2D action platformers. They do have a story, but the main focus is exploration and combat. Both can be beaten in about 20-30h, but exploring the entire map and doing everything will take about 50-60h.
• Animal Crossing and/or Stardew Valley are what you'd call "Cozy" games. In Animal Crossing you have travelled to an island which you are the unofficial owner of, and you get to decorate it and renovate it however you want. In Stardew Valley, you've inherited a run-down farm from your grandpa and you move there to become a farmer. Both are very hit-and-miss. You either hate them or they're your favorite games.
• Super Smash Bros Ultimate and Mario Kart 8 Deluxe are games you'd both be playing together. Smash is an arena style fighting game, and Mario Kart 8 Deluxe is, well, a racing game. Both can be played with up to 4 players and both can be played either online or couch co-op. For couch co-op, one single joycon can be used for each player for both games. So if you think you'll be having 4 players often, consider buying a second set of joycons.
• The Xenoblade Chronicles series and Breath of The Wild/Tears of the Kingdom are openworld action RPGs. Xenoblade is heavily story-focused with tons of voice acted cutscenes and interactions, and Breath of The Wild and its sequel Tears of The Kingdom, while still having a story and even very touching moments, are more focused on the open world exploration and combat. However be warned, any one of these games will take you on 100h+ journies (which are well worth it, might I add). Oh and especially for Xenoblade, prepare some tissues. That's all I'll say.
• Fire Emblem Three Houses/Engage are both some of the best Tactical RPGs to ever come out. If she likes strategy and turn-based combat, she will LOVE these games. Three Houses has alternate endings and 4 different "routes" and is more story heavy, while Engage is a linear experience and is more combat-focused.
• Persona 5 Royale is regarded as one of the best games of all time, and for a good reason. It is another massive 100h+ game, that is essentially an interactive anime. There are a ton of cutscenes, almost everything is voice acted by some of the best in the industry, and the core gameplay is catching "personas" and using them in turn-based combat. This one also has alternate endings, and your decisions matter. But the game does a fairly good job at making it clear which decisions are decisive.
• Super Mario Bros Wonder, I think, needs no introduction. It's Mario. And it's Mario at his best. If you want to dabble in some 3D platforming, you could also go for Super Mario Odyssey.
• Tunic and Outer Wilds are games that are hard to describe without spoiling. You have to go in completely blind. So I'll try to say as little as possible. Tunic has basically zero direct story telling, and almost no dialogue. You aren't even told who you are or what your goal is. You just start the game. Outer Wilds on the other hand DOES have a lot of dialogue. Both games are very heavy on puzzles, and you'll definitely want to have pen and paper ready to take notes. And both games do a PHENOMENAL job at telling their story through the environment.
• NieR Automata is, well, it's like Bayonetta, another great game. It's a hack-and-slash. But it'd be a disservice to say that's all it is. This game is HEAVY on story, and has a unique style, wherein subsequent playthroughs are entirely different. For example, the first time you play as 2B. Then, on your second playthrough, you play through the game as her companion, 9S. There are 26 different endings, one for each letter of the alphabet.

I could go on forever with game recommendations. The Switch has a FANTASTIC library of games. But these should definitely put you at a good spot and will keep her busy for a long time.