Motivational scaffolds can include some of the cognitive scaffolds under "Scaffolds for ensemble roles" and "Scaffolds for improvisation", because cognitive supports and social interactions can motivate students to persist in the face of challenge. Some motivation-specific scaffolds include a variety of student choice:
Sometimes encourage student musicians to experiment with a specific rhythmic pattern, melodic idea, or improvisation strategy. Most of the time, though, guide students through choosing their own ideas and strategies.
As time goes on, encourage them to arrange tunes as a combo. Leave room for students to choose what kind of background figures to play, and when, as well as how to start a tune and how to end it.
Select appropriate tunes for them to practice and perform, but offer choices there, too (e.g., you propose two tunes and they select one, or you select one and they bring suggestions for another). Listen to their opinions and preferences. And remember, it's not like in other ensembles when suggesting a tune means finding the right arrangement and spending a chunk of change -- all you need is a good ear or a lead sheet, and a little inspiration.