There is truckloads to see and do further north but since you've really only got a bit over 3 weeks it probably isnt worth going any further.
Maybe spend 4 or 5 days in Perth getting settled, seeing the touristy stuff (spending a rew hours in freo abd kiting at leighton wouldnt be such a bad day for a tourist) since you've flown from the otherside of the world and then head north stopping for a couple of nights at the better places (dongara etc) and then spend a week in exmouth.
It's a long drive so i think since its your holidays, your better off breaking it up for a night or 2 on the way home as well.
Gero will be windy but, well its gero