Concentration has been a long time favorite game. It requires observation and a good memory and of course a little concentration to be successful. Although the game is designed so two players play against each other it can also be played by a single player.
The object of the game is to collect as many matching pairs as possible.
A standard deck of playing cards
Two or more players. (You can play alone also.)
A large enough playing surface to lay out all the cards with seating for each player.
Lay the cards out face down in rows forming a large rectangle on the table or floor. Make sure the cards are not touching each other. They need to be able to be flipped over without disturbing any cards around them.
Decide who will go first. Customarily the youngest players starts the game.
The first player chooses a card and turns it over all while being careful not to move any cards around it.
The player then selects another card and turns it over. If the two cards are a matching pair for example two sixes then they take the two cards and start their stack. The player who just picked the matching cards goes again.
If the cards are not a match they are turned back over and it is now the next players turn.
The next player turns over their first card. If it is a match for one of the cards the previous player turned over then they try to remember where that matching card was and flip it. If they are successful at making a match they place the cards in their stack and choose their next card.
If the first card they turned over was not a match for one previously turned the player selects another card in hopes of making a pair.
If they do not make a match they flip the cards back over and play continues to the next player.
Play continues in this fashion until all the cards are played.
A players turn is not over until they are unsuccessfully at matching a pair.
When all the cards have been played the player with the most matching pairs is declared the winner.
To make the game more challenging use the rule that the pairs must not only match in rank, but must also match in color. For example a six of hearts and a six of spades would no longer count as a match, but a six of spades and a six of clubs would.
Don't stop paying attention just because it is another players turn. The cards that they flip over could be the match you're looking for.