Good question. Here are some suggestions:
<LIST>
<LI>- Twinkle Twinkle Little Star: This is a perfect first lesson in melody, since it has a very simple series of notes - CC GG AA G.</LI>
<LI>
- Five Little Ducks: This one has sweet lyrics about daring ducklings who waddle away from their mama. This one is perfect for sing-along with a series of hand motions - holding up five fingers, moving your hands up and down and the like.</LI>
<LI>
- Jack and Jill</LI>
<LI>
- Three Little Kittens</LI>
<LI>
- London Bridge is falling down</LI>
</LIST>
Speaking from personal experience, I fell our evergreen nursery rhymes make the best sing-along songs and kids enjoy them immensely.
