What are simple things that moms, who are too busy or tired, can do to stay fit?

Expend energy.: When you go to the store - park your car farther from the store - so you get more walking in. When you are bringing items from one floor to the next in your home, carry less - so that you have more trips (stair climbing). In commercial buildings opt to take the stairs instead of the elevator. Go on hikes or biking with the kids or take the dogs for a walk. Good luck.