Make sure everyone knows ahead of time that there will be an opportunity to do a little bit of physical exercise before the Pack Meeting so they can dress appropriately. Then, identify some people in the Pack who can put together exercise stations as part of the Pre-Opening. These people should make sure they understand the concepts behind the different exercises and how to do them properly. (For example, the "old way" of doing sit-ups is not the recommended way anymore. We don't need to teach them the wrong way to do the exercises.)
Get together exercise mats and posters that show how to properly do the exercises. Set up the stations around the Pack Meeting room with plenty of room to practice the exercises. Finally, have the "trainers" show people the proper techniques for the exercises and then let people practice them.
I don't think this should be a competition. There is too much risk of old fogies overdoing it and you don't need that situation on your hands. Just use it as an opportunity for people to learn and be encouraged to do proper exercise.