Typically a good rule is to think of sport activities that end in "-ing" - many of which have already been mentioned:
walking (power walking, not a stroll)
hooping (it's becoming very "hip" around here)
skiing (cross country)
Also structured cardio classes like Jazzercise (combines aerobic dance and strength training), zumba, spinning, etc.
If you take a class, be sure that your instructor is certified by an reputable fitness training program - one that will continually monitor and evaluate instructors. Don't be afraid to ask about their training and ongoing education. I've seen too many inappropriately trained fitness instructors that end up teaching movements incorrectly that could ultimately lead to injury.