Night one on Route 66. Seligman is where the Route 66 revival started — one street of old motels, gift shops and diners. What I’m after: drive-up room, neon sign, a bit worn but clean; the sign lights at dusk and a diner breakfast next morning.
Small independent places vary a lot from year to year — read recent reviews and check the neon still works before booking.
Options

Supai Motel, Seligman
First choice. The classic look: big neon sign, park at the door, walk to Delgadillo’s. Some recent reviews say tired and the sign was off — check first.

Historic Route 66 Motel, Seligman
Backup. Plain ground-floor rooms with the car at the door, at the west end of town. Cheap and simple.

Grand Canyon Caverns
The wild card: a single “motel room” ~60 m underground in a dry cave, west of town on old 66. Expensive, but the craziest night on the route.
More about the drive here in Americana & roadside attractions.