The Cottonball Composition is an activity I have used successfully with my young composers in elementary general music centers. Each set has activity sheets for the following melodic concepts: so-mi, mi-so-la, do-mi-so-la, do-re-mi, and do-re-mi-so-la.
Students work individually within their small groups, always with the option of collaborating with other "composers" in the group. And from there, it's just as simple as it sounds. Students compose a melody with cottonballs, sing it, and play it!
See my accompanying blog post at https://artsintersection.wordpress.com/2015/12/10/free-tpt-resource-cottonball-compositions/