For the average person, with or without Down syndrome, adding small amounts of aerobic activity on a regular basis makes a difference. Even small changes in daily activities can be beneficial. Here are some ideas for adding aerobic activity. Walk or bike to activities that are in your community. Don't catch them, just chase them around.
Parents think of swinging and climbing the play structure as being active.
It's not aerobic activity, except when running between structures. Go on a long walk, hike, or bike with a friend once a week. Join a local recreation facility. Join a walking club. Coming up with ideas to increase activity is the easy part.