In one of the extra programs that I teach in at my uni, we have an "activity day" periodically. For groups of less than 8, it's very easy to come up with something to do. However, what about those groups of 8-15? A bit more of a challenge. Something that works really well is "Spoons" or "Board-Marker." Bring enough spoons or markers so that you'll have the same amount as people, minus 1. Then, you need a deck of cards. Sort them so that you have 1 set of 4/person, plus 2 extra for smaller groups or 3 extra for the bigger ones.
Have the students sit around a table in a circle. Put the spoons spread out in the center of the table. Deal each student 4 cards. You'll have a few extra cards in your hand. Give one more card to the person on the left. They look at it, and either keep it or pass it on to the left. And so it goes around the circle. You keep dealing out all the extra cards and it's a speed game. When a student has a matching set of 4, they pick up a spoon. Then, all the students try to get a spoon. The student that doesn't have one is out. Keep reducing the number of spoons and cards as more people are out. You'll eventually have 2 winners. Give them a point and play another round.
1. Make sure students only have 4 cards in their hand at a time.
2. If you have a bigger group of say 12 people, reduce the spoons by 2 or 3 each time to speed the game up. That way, if you're out in the first round, you only have to wait a few minutes until you can play again.