Pyramid is a quick solitaire game about simple sums. Instead of building columns you take cards away in pairs that add up to 13, and the goal is to take apart a pyramid of 28 cards. A game lasts a few minutes, but the order you remove cards in matters more than it first seems. It is free here, with no download and no signup. Every deal has a number you can share, and Hint and Undo have no limit.
The deal lays out 28 cards face up in seven rows: one card at the top, two below it, and so on down to seven at the bottom. Each card is partly covered by the two cards in the row below it. The other 24 cards go face down into the stock.
Before the first move, look at what each pair would uncover. Taking a pair from the bottom row is only useful if it frees a card above it, and a card is freed only when both of its covering cards are gone. Prefer pairs that work on the same part of the pyramid over pairs that nibble at two different corners.
Count the ranks. There are four cards of each rank, so there are four of every partner. If a 6 sits directly on top of a 7, those two can never be removed together, and you will need another 7 for that 6 and another 6 for that 7. When the pyramid has several cards of the same rank, decide which one each partner is for before you spend it. A 9 used on a waste 4 is a 9 that cannot free a 4 in the pyramid later.
Treat the stock as a resource you can learn. After the first pass you have seen every card in it, and the order never changes, so on the second and third passes you know which card is coming next. Kings in the stock cost nothing: remove them when they turn up. If you are stuck, Hint points at a king or a pair you can remove, or at the stock when there is nothing else, and Undo takes back as many moves as you like.
Many Pyramid deals cannot be cleared even with perfect play, so a lost game is not always a mistake.
For longer games, try Klondike Solitaire, FreeCell or Spider Solitaire.