Located in the center of Hellfire Peninsula, Hellfire Citadel is the first instance you're likely to do in Outland.

Coilfang Reservoir lies below a large structure in the central lake of Zangarmarsh

Auchindoun lies in the center of the Bone Wastes in Terrokar Forest

The Caverns of Time can be found on Azeroth, in Tanaris.

Tempest Keep consists of several floating structures, a little east of Cosmowrench in Netherstorm.