Spring and late summer. The older I get, the less I can tolerate crowds and really hot summer days. I find the brats are back to school in America earlier than they are in Canada so it ends up the perfect time to visit. Spring can be iffy for crowds with school trips/band trips, but the weather is more pleasant than at the height of summer.
If it's Florida or a park in a hot climate, late winter is ideal, after spring break is done but before the school trips begin.