just realised I need to book flights to WA for the family by midnight tonight to take advantage of a points deal for a family trip over christmas/ new year (wife, 2 kids 11 and 13)

I can stay at my sisters (who has just moved over so no help in planning) in kallamuna and am willing to pay for accomodation in other spots and hire a car etc.
I was originally thinking three weeks - WE would like to see as much as possible and have never set foot in WA.
Or should I make it longer? Obviously perth/rottnest freemantle, possibly margaret river area all from perth? Then several days to see albany area. Then geraldton and coral bay / exmouth/ ningaloo
Am I trying to do too much? I might not be able to return for a long time. It's a long way from North Queensland.
So far the only things I really know about are rottnest and I hear the whaling museum at albany. Any opinions or suggestions welcome