Jay’s right, and out of his choices Toodyay Boulders is the best. Not only it dries the fastest as on top of exposed hill but if the weather turns nasty you are back in car in 10min. Plus the main boulders have a built in rain shelter.
Besides Toodyay any other granite boulder with no runoff on top. But avoid “soft granite” (Avon Valley NP, Bald Hill), it gets water soaked and crumbly….funnily enough….