The crew! Both versions of it allow for replayability as you have different objectives every game. It is a trick taking game, so they might be a bit familiar with it due to other games like bridge. And it is cheap. Only thing that might take them a bit to get used to is that it is coop.

Another simple one would be take 6 (nimmt 6).

• Bohnanza is a good recommendation. It's very easy to learn and doesn't fall into the usual brackets of shedding or trick taking.
• Scout is my favorite combo shedding game (ie. you're shedding multiple cards at once). The way you're always either drawing or shedding keeps the play much more dynamic than in Daihinmin or the Great Dalmuti
• Frantic from GameFactory isn't widely available but in the EU you can probably get it. UNO on steroids. More exciting special cards and you can play them against any player, not just the one following you.
• Tichu a team based trick taking game that has a lot of fans around here. I personally have never really taken to it. It's fine but I don't get the rabid fandom some have for it.
• The Crew is ridiculously difficult with 5 and cumbersome with 2. Would only recommend it in between. But there it is a very clever game.

Bohnanza is a modern classic.

Scout is supposed to be great.

Tichu is a bit more complex (and somewhat similar to Doppelkopf, which as I am just learning is apparently rather specific to Germany).

Arboretum is also nice, somewhat similar to Lost Cities.

We often play "6 nimmt" with our Grandma. That is always a blast. Simple to learn, quick to set up and a lot of Schadenfreude from my Grandma when she tricks somebody into taking cards!

Exploding Kittens is always a winner when I break it out.

I would also echo the Crew is great, especially for people that are familiar with traditional card games.

Sushi Go, Hanabi, Coup

Our current favourite The Crew was already mentioned.

Phase 10 used to be the staple to bring out at gatherings, both with family and friends. I personally have outgrown it a bit, but it's easy to learn and it's probably the game in our household that has been played for the most hours.

• Sea Salt & Paper is set collection with some rummy elements. Some simple but fun card abilities mix it up. One of our favorites. Available in the EU (Philibert, probably others too)
• in Cabo you are trying to minimize the value of your face down cards. Has a memory element but not too bad. Widely available.

"No Thanks!" is super easy and fun.

But I've also found with a lot of oldies that they actually like playing the same old games. They often have more depth and complexity than you might think at first glance, and it's difficult to learn a new game of similar complexity later in life and might be dull to swap out for a simpler game. Like, if some whippersnappers were to go "granny, put those delvers and cantrips away and come play some fun Cards Against Humanity with us" I'd probably immolate myself on the spot 🤷🏻‍♀️

Card games that aren't too hard to learn:

• Star Realms / Hero Realms
• Yomi
• Ascension
• Keyforge
• Love letter
• One Night (needs a phone app)

Uno has many variations. My current favorite is Uno Flip.

Cribbage is often paired with a board & pegs, though doesn’t technically require one as it’s meant for tracking points and sometimes victories.

By card game, is that with a standard deck? Or something custom like “Monopoly Deal” which is one of my favourite card games!

I had fun with regicide: https://www.regicidegame.com/ You can buy it or just download the rules nad play it with any old poker deck.

With the right group of elderly, it could be fun

Point Salad has been a big hit and I’ll back up what others have said about Bohnanza.

I’m tempted to recommend Cat in the Box as a new take on trick taking but I haven’t had the chance to introduce it to non boardgamers yet. It’s a very cool concept but the abstraction of suits could be a bit much for some folks.

31 is simple, and you only need a standard deck of cards.

Checkout Dutch Blitz, it's kinda like multi-player solatire. Up to 4 people.

The games I've had success with elderly players are:

Lotus (play petal cards to complete the flowers)

L.L.A.M.A. (colourful, be the first to play all your cards)

and

Illusion (visual game, which card has the most of a chosen colour)

Other possibles are: Calico (cats and quilts)

or if a pirate theme is acceptable, I'd highly recommend Port Royal(pirates, ships and treasure)

Also, Morels (find mushrooms in the forest) or Kanagawa (make Japanese paintings) have great themes and solid gameplay.

I'd suggest L.A.M.A.. It's been a hit with non-gamers (because it has simple rules and great flow) and gamers (because after a few rounds you realize there's more to it than getting lucky).

Coloretto is one of my favorite card games. It's simple, but there's a lot of fun strategy in balancing 'protect my interests' and 'watch out for others getting something too good or taking what I want' with the limited single action per turn

Medici the card game. It's basically blackjack with a twist.

Nyet, a tactical trick-taking games. Best at 4 players.

Also, I love Cold War: CIA vs KGB. Just take care of the errata. It's blackjack with secret goal. It's a duel game, but hand-free.

Something a little more different but still a card game: High Society. It's a simple biding game with simple rules. I like it a lot, but first I'd go for the classics like The Crew, Bohnanza and maybe Scout also..