This is based on, and very similar to, the game of Concentration you probably already know and love. With a SET® deck it's quite a bit harder.
One deck of SET® cards, and enough room to lay out all 81 cards.
Lay out all 81 cards face down. Decide who goes first in some reasonable manner.
Players take turns turning over three cards of their choice in hopes that they will be a Set.
If the three cards turned over are a Set, the player gets to collect and keep them, and turn over three more. Otherwise, the cards are turned back over and the player's turn ends.
The game ends when all players agree that the remaining cards contain no Set.
Whoever collects the most Sets wins!