Daniel Spatz. Simple, no extra equipment needed, and you can work on it as long as you have time for.
The goal is simple: ask your player to complete three rallies, five times in a row. If you have time, keep working on this until the goal is met. If you have a large group or limited time, keep track of how many times the student is able to complete the task. Once? Twice? None? Either way you have a simple but achievable goal for them to work on.
This would also be a great warm-up activity between players for larger groups. Have all working on this as a singles drill. First court to complete it successfully wins, and signals the end of the warm-up period.