Visit the mall and see how many times you can walk around it. Local bike clubs and many other organizations sponsor bike trips appropriate for the whole family. Except in winter, community biking trips, races, and rides for charity abound. Enter a local foot race. There are fun runs and walking races sponsored by clubs, churches, and charities.
Participants are of all ages.
You don't have to win. Let your kids see you having fun with sports. Then go practice with them. The idea is fun, not winning. Plan a family adventure vacation.