This is from a flash animation of Asuna (from Negima), but I've seen this in a lot of shows, especially during the ending themes. Most recently, the ED of Recorder and Randsell Mi.
Is this a real dance or just something that's easy to animate?
The instruments are called Maracas which are similar to shaker. Similar to how some singers use tambourines, other use maracas when they are singing often dancing while playing in rhythm to the shake.
You can often see backing vocalists use such instruments to appear busy when they are not required.
Here's how to dance with Maracas.