My reccommendations for wildflower exploring would be the Mt Adams Wilderness, and especially Bird Creek Meadows, at the southeast corner of Mt Adams on the Yakima Indian Reservation. One can still probably find 50-60 species in bloom at the latter location. Further north, one could try the Goat Rocks Wilderness, and Mt. Rainier National Park. Sunrise, on the northeast side of Mt. Rainier can be an especially rewarding wildflower viewing area, as again, 50-60 species should still be in bloom much of September. Further north, I am somewhat unfamiliar with what might be in bloom, but would guess that The North Cascades National Park and the alpine areas of Mt. Baker would offer good wildflower viewing through September.
Beware at this time of year of the ever increasingly aggressive yellow jackets, and although the fire danger in the cascades is not excessively high at this time (Sept. 7), the drying winds of September and October can change that in a hurry!