Monkey Mia a day drive?
Total 860.25 km, Total 17 Hrs 56 Mins.
So, I guess technically you COULD
do it in a day, but not in reality, or safely.
Gee, these Eastern Staters have no idea
I'd go with a previous poster and go for the Pinnacles (5+ hours according to whereis) or Margaret River (a 3 - 3.5 hour drive with no stops from regular experience), but I reckon Pemberton would be better. Margaret River is, IMHO, over-rated, but the region is what you should see.
Rottnest Island, haven't been since I was a kid, but people still like it. Depends on the weather, if like today, don't even consider it.
Make sure you catch the Perth metro beaches, Kings Park, the view from a tall building (unfortunately none have public viewing), if I was home when you were here, I'd off to take you up to L25 of where I work for a great view, but I'm in Brisbane.
Anyway, enjoy your trip!