On small pieces of paper, write down chores and body weight exercises. Throw the papers into a couple of hats and have everyone pick one of each. Maybe Dad gets to clean the bathroom and do a set of squats every few minutes until he is finished, while one of the children is cleaning the kitchen and doing forward lunges, etc.

Organize a Family Competition:
Have each family member pick an exercise at the beginning of the week and do as many reps as they can. Then train throughout the week with the goal of improving by the weekend. The family member who has the highest percentage increase is rewarded with a prize. Keep the focus on the fact that everyone is improving.

Establish a Family Fitness Night:
Designate one evening as family fitness night. Each week, a different person designs the family workout that you will all do together (swimming, rollerblading, walking, Frisbee golf, basketball, etc.) Everyone gets to do something they enjoy, and it will help to bring variety to your workouts. 

Incorporate Exercise into Daily Activities:
Establish family fitness rules: exercise during commercials every time you watch TV, stretch before bed each night, etc.  Purchase a few inexpensive items (jump rope, resistance bands, stability ball, dumbbells, etc.) and rotate them through the house on a daily basis. Treat them as scavenger hunt finds; for example, if you find the jump rope placed in the garage, you have to use it for one minute. Next time you may find it on the porch, or in the basement.

Exercise with a Goal in Mind:
Some prefer to exercise when they are training for a specific event.  As a family, sign up for an event that everyone can participate in and train for.  Organize and conduct workouts that would help train/prepare everyone for the event.

Sources:
http://www.sparkpeople.com/resource/fitness_articles.asp?id=382