String singing is another variation of "who sings". The idea is to tie different colored string or yarn together in varying lengths, roll the string into a ball and put this into a can. Then pull the string out through your fingers. Different children sing according to the color changes. This is a fun way to review a song several times.
The picture shows the can for boys and girls singing. The blue string instructs the boys to sing, the pink indicates girls singing and the green color represents everyone. I've made the string start with green so that everyone begins singing. The children have to watch closely and keep the song going in their mind to be ready to sing when their color comes up.
One advantage to the string idea is that because you control the length of the string you have some control of the timing. Even though the children are manipulating the string, the singing can be more smooth and the review is more effective. When the children use the boy/girl puppets, for example, they love to shift quickly from one to the other and it can become a little confusing. While this is fun for the kids, it obviously doesn't review the song as well.
I also have a ball with green, yellow and red string. The green yarn indicates that everyone should sing, the yellow yarn tells us to hum and the red yarn says to stop singing. The piano continues to play, so the children need to keep the song going in their mind in order to come back in on the right words.
There are probably other ways to set up the colors. If your primary assigns class colors you could have the classes sing in turn.