Iquique is a long narrow city squeezed between the coast and the cliff. Most of what visitors care about — the historic centre, Playa Cavancha, the Zofri mall — lies along the coast and is connected by frequent colectivos (shared taxis with fixed routes) and micros (small buses). Colectivos cost around CLP 700 and run constantly; flag them on the main avenues.
The old town is compact and walkable. For Humberstone and other inland trips, either rent a car (several agencies at the airport) or join one of the half-day tours sold out of agencies along the Cavancha seafront. Taxis are metered and reliable.