I love Canyonlands and it's a vast national park. However if you're looking to hammock at willow flats, you're probably out of luck as there are little to no trees.
Devils garden is one of the main attractions in Arches national park. The camp ground is located to the right when first entering a parking lot. It is by appointment and I believe it's 30 a night. Very clean grounds and facilities and it's one of the few places you're allowed to have fires in the prk. Be aware that you must sign up for a spot far in advance as they fill up quickly.
I love the dense trees surrounding this campground. It's a great place to hammock and hike. Scout falls (while not overly impressive) is about 1 mile into the hike. Make sure you camp in designated spots as fires are prohibited everywhere else.