It really depends on where you'll be and what dates in September. Late September will have gorgeous aspen viewing in certain areas but not others.
But here are some general must see areas:
Great Sand Dunes National Park
Rocky Mountain National Park
Mesa Verde National Park
Colorado National Monument
+1 for Flatirons
@Dug-E-Fresh im not sure how crazy busy this would be but dream lake in rocky mtn natl park was a really cool bike we did while in Estes park. It was a couple years ago so 🤷♀️ But it was definitely a cool hike and destination then
My preference would be to list some of the easier 14ers to climb. But since you have said day hike, I have to recommend Garden of the Gods even though it's popular. It's near Colorado Springs, a nice secondary place to visit if you are based in Denver for your vacation. You can always take the train ride up Pikes Peak, or drive up.