I adapted this idea years ago and cannot remember where the original idea came from. But I use it often in Junior primary.
The idea is that we are going to "measure" how well we have learned our newest song. I use a very large flower vase. With a marker I mark a line near the top. I have then also have a pitcher filled with water. What is so fun about this is I use food coloring to color the water. You can use a color that matches the song. This time we were singing Beautiful Savior, so I used green to represent all the beautiful things he has created (meadows, woodlands, flowers, etc).
I broke the song up into sections. Aster we sang a section I would rate how well they sang it. I then chose a child to come pour colored water into the vase a bit. (I would just tell them when to stop). This continues until the vase is almost full. Then to reach our goal we stand up and sing the song through in it's entirety. I ask them before if they think they are going to make it to the top. The suspense is so cute and they get so excited! Of course they always make it to the top. What's great is they really earn it!