Feel free to be as creative as you would like. Many schools choose to invite local celebrities to the event and play music to accompany the activities. Organizers of local events can choose to get the whole school involved, or just selected classes. The majority of schools will get the entire school population outside on to a play field, put on music, and have everyone in the school have some fun by moving and exercising together. If it rains, have a backup plan.
Some schools have the student body go to the gym and do aerobics and calisthenics.
Another option if it rains is to pump music over the PA system, and have the classes exercise in their classrooms. Governor's office may also be read. The motivation for the participants is to know that there are children and adults all over the world exercising and moving along with them. Assembly programs with different health, fitness, nutrition, and lifestyle related themes. Last year I sent a great media scrapbook to The President's Council on Physical Fitness and Sports with press, programs, and accolades you sent in.