Letter of Recommendation

I have a graduate degree in public health, but I continue to work at the bedside. I am spending the summer volunteering (approximately 20 hours per week) as a RN in a STI/HIV clinic in order to diversify my nursing experience. I think the executive director and the other part-time RN are surprised that I have mastered the basic job requirements as quickly as I have. I would like to request a letter of recommendation from the executive director. That is an appropriate request, right? Because I am looking to transition to public health in the coming year or so, should I ask them to highlight specific areas/strengths that I/they perceive as assets to, say, a public health department OR simply let them write whatever they'd like? Thank you for your advice.