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Whiteman park for the day! has a train museum, tractor museum and motor museum. A tram to ride, old steam train rides, lots of paths and you can hire pedaled go carts there, and visit caversham wildlife park while you're there. Plenty of free bbq's to use too. Plus a little pool to cool off in but it's only very shallow probably more for little kids to splash around in but nothing stopping older kids playing there too if they want. But lots to explore, worth a day trip
1.5 hours from Perth in which direction ?
Are you going 1.5 hrs from Perth city itself? Past mandurah is peel zoo which has lots of Australian animals, my kids really liked it they were 10,11 at the time. Aqwa was interesting.