Beginner students tend to think about melody more than harmony. Therefore I have been experimenting with a beginner ensemble approach that takes two melodies that a student already knows (or are simple enough to learn easily), and puts them together as a duet. In this way, students begin by already knowing both parts, and can focus their attention on how the two parts fit together. They can do this without needing to follow a score, if both melodies have already been learned by ear.
A note to teachers: the following scores can be transposed into different keys depending on the instrument you are teaching. If you discover any other tunes that fit together, please let me know and I will add them to the list and we can all share them together. Thanks!