If there is a pack of cards to hand then there are a multitude of games to be played. Like jokes, I can never remember a card game when I need one so this post is in no way altruistic; it is a list that I know I will use myself. Adapt each games as you need to, if you have the luxury of an extra person encourage them to assist the player with dementia as much or as little as is necessary. Sometimes all that is required is a gentle but regular reminder of the rules or to keep an eye on whose turn it is.
If the person you are playing with has a poor sight you might want to invest in some large print playing cards or extra large playing cards.
If holding a bunch of cards has become difficult you can get attractive wooden card holders
Of course the most simple and well known game. You can play this with 2-multiple players. The dealer deals the entire pack of cards out face down between all the players. Then, starting with the player to the left of the dealer, and in turn, each person lays down the top card from their pile into a new pile (the discard pile) in the centre of the table. If the card being laid matches the card laid down by the previous player all players must say "Snap". The first person to say "snap" wins all the cards in the discard pile and play starts again. The winner is the player who collects all the cards.
The dealer deals all the cards. If there are two or three players, each player is dealt seven cards. If there are more people taking part, each player is dealt five cards. The remaining cards are placed face down in a pile. This is the “fish pond.”
Each player sorts their cards into groups of the same number or suit (i.e. group of threes or group of kings), making sure not to show anyone. The person to the left of the dealer starts the game by asking any another player for cards that will match his hand. For example, if they have two threes, she will ask the other player for threes. If the other player has these cards, he must hand them over. The same person continues asking the same player for more cards until the player does not have the cards he wants. If the player does not have the right cards, he can tell the requester to “Go fish.” The requester then has to take one card from the “fish pond.” The player who told him to “Go fish” becomes the new requester.
Anyone who collects all four cards of a set (i.e. all four eights or all four Queens) puts them face down in front of him. The winner is the first person to have no single cards left, only complete sets. If two people run out of cards together, the player with the most sets wins the game.
This is a two player game. All cards are dealt to the two players and kept face down. Neither player must look at their cards. Both players turn over the top card of their piles and put them face up in the centre of the table, beside the other player’s card. Whoever has turned over the highest ranking card takes both cards and adds them to the bottom of his pile. This continues until two cards of the same value (i.e. two sevens) are put down together. The game is now in a state of “war.” To continue, both players take two new cards and put one face down on top of the card they have already placed in the middle and one face up. Whoever puts down the higher ranking face up card wins all six. The game is won by the player who collects all of the cards.
Beggar my Neighbour
Deal out all the cards between the two players.
Each player takes it in turns to turn one over from the top of their pile and put it on the table between them (as with snap!) If you turn over a Jack, Queen, King or Ace, the other player must put down more cards as follows:
Jack - one card
Queen - two cards
King - three cards
Ace - four cards
If no picture cards are turned over whilst this "payment" is being made, you collect all the cards from the table and put them at the bottom of your pile.
However, if a picture card WAS turned over, your opponent immediately stops their 'pay out' of cards, and YOU have to pay them by putting down one, two, three or four cards, according to the rank, as above.
The winner is the player who collects all the cards.
I would recommend printing / writing out the "payments" as a visual reminder during the game.
Pelmenism / Concentration / Match
Maybe not a card game in the truest sense of the word but even so, one worth mentioning. There are some nice, adult themed match games on the market, or if you are feeling creative you could make your own.
You place all the cards face down on the table and each playing takes a turn to turn over two cards. If they match the player keeps the matching pair. If the cards don't match the player must place the cards back down on the table in the same place. The aim of the game is to remember which cards your opponents turn over and where they are so that you can match pairs when it is your turn to play.
This game can be adapted to the short term memory impairment of the people playing by reducing the number of cards you use.