Congrats to you and your wife, @DocT!
If you haven’t been before, traveling around Colorado and New Mexico would be great that time of year. You could catch the Aspens turning color. Telluride, Aspen, Taos, Santa Fe are all great just to name a few spots.
Since you are not leaving until September, I would nix Yellowstone and Glacier. You are apt to get cold, rainy weather up there that late in the season.
But the aspens will be golden in Colorado. The Telluride area is beautiful that time of year. Telluride is just a couple of hours from Moab, Utah. From Moab you can see Canyonlands and Arches national parks. Spectacular. The yellow rabbit brush will be blooming and its beautiful against the red rocks.
Just want to add - we were in Colorado late September/early October last year and just happened to luck into being in Aspen during their annual film festival. It was an unexpected treat to catch a few films. That is when we first heard about the Oscar winning film Room.
St Louis Gateway Arch
I spent 9 months on the road photographing fire stations when I was 24. Highlights of the trip when we weren’t doing our job: eating at Calvin Trillin’s favorite restaurants in New Orleans, The Grand Canyon, Carlsbad Caves, Bryce Canyon, Caltech campus, Los Angeles mid-century modern architecture (Neutra, Shindler etc.) The Madonna Inn in San Luis Obispo, Napa and Sonoma CA, hiking in the Sierra Nevada mountains in the late summer, Joshua Tree in the spring, The Coastal Highway between LA and San Francisco, Yosemite, Nelson-Atkins Museum in Kansas City, Arthur Bryants Ribs also in KC, U of Chicago Campus, Frank Lloyd Wright walking tour in Oak Park, Cinncinati Chili, South Union Shaker Village in Auburn, KY, Freedom Trail, Boston, Maine Coast, White Mountains NH, NYC. And that’s not even covering the truly weird stuff like the Lurleen Wallace Museum in Montgomery AL. (Which apparently closed ten years ago.) There’s stuff to see everywhere and plenty that I still need to see (Rushmore, Yellowstone, Alaska…)