September is an ideal month for many parks, but I recommend visiting these five in particular: Carlsbad Caverns, Crater Lake, Grand Teton, Redwood and Sequoia.
Besides the magnificent cave, the top experience at Carlsbad Caverns is watching the bats in the evening, and September is best for that. Arrive at least 30 minutes before sunset to get a good seat and listen to the ranger talk.
Crater Lake is surprisingly tricky to visit. The rim road is frequently closed in sections, and the park gets crowded and can be too hot in summer. September is by far the best option, as snow starts falling in October and the road is closed until the end of June.
Grand Teton is simply beautiful in September, with aspen foliage, elk rutting, fat bears, pronghorn, and less crowded scenic drives. Water levels can be low, especially later in the month, so if you want to raft or kayak make a reservation for early September. Plan ahead as the area has gotten extremely pricey.
Redwood has a long season along the temperate California coast, but I like September best. The hiking is easier because it’s dry, the elk are easy to spot, and there are hikes that go through lush green areas like Trillium Falls to give you a sense of flowing water. And after hiking a bit, I had the trees to myself.
Sequoia is another California park where you want to wait until the crowds of summer have passed. It will still be busy, but the weather is still nice and a bit better than October.
Timing is very important for having the best park visit, and these particular parks pay off best in September.

