We're pretty lucky at our centre, the children love performing and participating in group activities. Most of the time, the children choose an activity they would like to do in our transition time and ask us before afternoon tea is finished.
More often than not, they want to have "share time". They choose what they would like to show the other children, stand up in front of everyone, and perform/display their news. We have a group of girls from different years who like to practice a dance performance at lunch time and perform it for us in the afternoon. Some of the other children bring in photos from their weekend or a slideshow of photos on their iPads.
Sometimes we hold competitions - the children have to stand up and tell us one thing that they learnt in class that day and teach it to the other children. We use the Woolworths Ancient Animal cards as prizes because they are in hot demand at the moment - and they're also free! We all collect them from our shopping and bring them in.
Another idea is playing a quick game. When we have more children and it gets difficult to have share time (we'd never get off to program time!) we like to play Zip Zap. We get all the children to stand in a circle and have one child in the middle. The child in the middle puts their palms together, points and someone and says "Zip!" (we also like to stress that the children don't hold their hands like a gun, and that the "zip" is a ball of energy). The person who is "zipped" has to bop down and the children next to them have to try to "zap" each other first. The slower child sits down. This continues until there are two children left, they stand back-to-back and start walking until one of the educators says "zip". The fastest child to "zap" wins.
That's just an example though, you could obviously think of all different kinds of group games. We actually have a book of group games that I often choose activities out of, but I'm sure you could find lots online. We've also done what Lorina suggested and played a trivia game with buzzers and points. We do a lot of different activities, it just depends on what children we have on the day and what they feel like doing