Where to go on family vacation?

I am trying to figure out where to go with my family on vacation in the December/January 2011-12. We have narrowed it down to a South American trip, Spain, Greece, Morocco, South Africa, Israel, Australia, and Central America. My family consists of a 18 year old boy, a 20 year old girl, and two parents over the age of 50. We cannot decide which destination to choose! Help!