RomeoMike writes:

Marble Canyon airport sits above the part of the Grand Canyon known as
Marble Canyon, but it's not in the Park Boundary or down in the Canyon.

It is indeed in the canyon, which starts at Lee's Ferry.

It's not inside the park, but it's inside the SFRA, which is bigger than the
park (the park covers only a fraction of the canyon).

There's also Phantom Ranch, but it cannot accommodate fixed-wing aircraft as
far as I know.