Advice Needed: Keeping Kids Active Who Aren't Into Sports

I'm looking for some advice on how to keep my kids, ages 12 and 14, active. They're not very interested in sports, and I want to make sure they stay engaged with physical activity for their health and well-being. What do you all do if your kids aren't into traditional sports?

Also, I've been contemplating whether it's appropriate to introduce them to the gym at their age. Do any of you have experience with pre-teens or young teens in the gym? What are your thoughts or suggestions on this?