Make strict rules regarding the amount of time your kids can spend watching television or using the computer: an hour or two every day maximum. Adhere to these rules rigorously so your children are forced to come up with other ways to entertain themselves.
Include your children in chores around the house. Things like watering plants, taking the dog out for a walk or vacuuming will give your children responsibilities and make them spend their free time in a productive way. It will also help them acquire certain skills.
Be a good example for your children. If your kids see you enjoying being active, they will follow your lead and be more active themselves. Young children in particular are heavily influenced by their parents and like to mimic them.
Ask your children what they would like to do. Some children are more interested in sports while others might be more keen on learning music or languages. Consult your child on which extracurricular activities appeal to her and listen to her wishes.
Do things with your child. If the child connects activities with spending time with his family, this will be additional motivation for him to get engaged. Take family trips, read with the child or go for walks with him.