September is great time to go, Summers are blisteringly hot. I would recommend waiting until after Labor Day and kids are back to in school and college classes have resumed to thin out the tourist hordes.
There are tons of very nice Air BNB rentals in Moab. (D2 has a friend who is a doctor there and she can’t afford to buy a house! The real estate market in Moab is insane.) Those will get you a kitchen to cook in and access to washer and dryer.
(You will probably want the washer and dryer because the red dust gets on everything!)
Entry to Arches National Park requires a prior reservation between April 1 and October 31. Your entry is for a set time on a particular date and has a 1 hour window for use. Tickets for September will go on sale June 1 so you’ll want to make your reservations as soon as know what date you’ll be going. You will need to make a Recreation.gov account to purchase your tickets.
I highly recommend signing up for the tour of the Fiery Furnace while you’re visiting Arches.
The hike to Delicate Arch can be strenuous and the trail is pretty heavily used, especially near sunset because everyone wants to see the arch with the setting sun in the background. The hike is hotter at the end of the day because the sandstone has heated all day in the sun and radiates the heat back at you. Hiking in the morning is cooler.
There are 3 sections of Canyonlands National Park–
- Island in the Sky which is on a plateau above the Colorado and Green Rivers, There is a famous jeep trail that circles the plateau, the White Rim Trail. (D2 and SIL mountain biked the whole 100 miles of last weekend.) Spectacular views, but it does require a high clearance vehicle w/ 4 wheel drive. Island is north-west of Moab and past Dead Horse Park.
- Needles is south of Moab and you;'ll pass the turn off for it if you access Moab via Rte 191. Needles is probably the most accessible for 3 sections of Canyonland. You can do an “out and back”, stopping at the visitor center to go out and do some day hiking. There are several sand stone arches as well petroglyphs and early puebloan ruins in the park, but getting to them will require either hiking or having a high clearance car + walking.
- The Maze is on the west side of the Green River and is the least accessible section of the Canyonlands. It has minimal roads and all of them absolutely require a high clearance vehicle with 4 wheel drive. Stunning scenery, though.
No section of Canyonlands requires reservations unless you plan to stay overnight.
If you want to get into the backcountry, but don’t want to drive your car or don’t want to rent a high clearance 4 wheel drive car, there are lots of 4 wheel tours you can take. There’s also white water rafting on the Green River.
There are lots of arches outside of the Arches National Park, but getting to them will require some hiking to get to. But check first, the BLM has been closing some trail to help prevent erosion of the landscape and to protect wildlife from people doing stupid things.
EDIT to add–if you can spend the night or any part of it in the backcountry of either Island or Needles. you can actually see the Milky Way. The only time I’ve seen it have been either in Canyonlands or Yellowstone/Grand Tetons. When Halley’s Comet last made its pass by earth, we took the kids and went with a bunch of friend way, way out to the backcountry of Needles District, past Elephant Hill. You could see Halley’s with just your eyes. It was pretty special. We were out there for 3 nights with telescopes.