This is awesome. This is my 10th year of teaching health and phys ed. My first year I did Kindergarten PE, Sophomore PE, and a liftime fitness class called Just Fit with the Home Ec teacher. Next 2 years K-4 PE. Next 6 years High School Health and PE. Thisyear I was bumped b/c of budget cuts to K-5 PE. Best advice is keep it simple. Find a way to relate the material to each grade level. Hands on activities, stories, authentic activities (such as your example of choosing) are great. This would be overwhelming to me b/c of cutting the info down to their levels. I have a hard time doing that with a whole unit on nutrition with high school let alone elementary kids.
You probably can't do this, but I play a tag game in PE where I choose 3 students to be superhero villains. Donut Man, Ice Cream Girl, and Pie Guy. I give Donut man a ring toss ring, Ice Cream Girl a ball Scoop, and Pie Guy a frisbee. If a student is tapped by them they have to come to me and give me an example of a healthy food they could eat instead. They love it. If I think of anything more Classroom oriented I'll let you know.