This is a different kind of challenge, but one where you take the lead and choose your favourite kind of exercise with your favourite people.
Invite a group of friends or family to take part in a virtual exercise of your choice. Choose any kind of physical activity you like - yoga, Zumba or your own version of a PE lesson are popular choices but the decision is yours. Either lead the session yourself, or stream a recording from the internet for everyone to follow at the same time. Set up a Just Giving page and ask people to donate what they would have spent on a gym session or exercise class.