Have each person take a card at look at the playing card number. Then, as quickly as possible, see how many groups can be formed that add up to as close to 21 as possible. However, this is a dealer loses version, so every "group" must be a winner. That means that the goal is to use every person and to have every group be as close to 21 as possible without any group going over 21.
Give it a couple tries. Usually on the first try a few groups will form that equal 21 but then you are left with a group of all face cards or a group of 3s and 4s. Keep time, reshuffle, and see if your group can come up with a strategy to beat their time. Keep track of how far from 21 each group is and how many people are left out. Debrief by asking what strategies worked the best. What made it more or less difficult. If you wish to make this activity more difficult ask your group to do all of the above without talking or showing their cards. But always remember, HAVE FUN WITH IT!