Coon suggested hunting the high Cascades wilderness areas. In late September, I would avoid those wilderness areas that are open to the modern firearms High Hunt which closes on September 25th. The Sawtooth Wilderness is not open and would be a decent choice. There are a few good high country summer range areas that are not designated wilderness and are open only to archery hunters in September. The High Hunt areas open to archery hunting on September 1st and would be a good choice during the week before the elk archery season opens.
What's wrong with hunting the Alpine Lakes Wilderness (Alpine GMU 249)which is open for deer, bear and elk (Sept. 8 - 21) "any elk"; in reality nearly all of the elk taken are mature bulls (8 to 12 percent harvest rate). The deer High Hunt with rifles start on Sept. 15th but not many hunt the elk hot spots.