There are five sites in reasonably close proximity: you can visit all five in a day if you have a car:
Nemrut Dağı*
Arsameia
Eski Kahta*
Karakuş tumulus
Cendere bridge*
Details and photos are on the following pages.
The first four of these were built by the Commagenes, the fifth is a Roman bridge from around 200 AD. There’s a wonderful mountain road leading from Arsameia to Nemrut Dağı: you might overheat your engine on the way up or suffer brake fade coming back down, but worth it for the views.
*: The photos of these sites are from autumn 1983 and spring 2021.