The Ruins of Banteay Srei
One of the smaller Angkorian temples, Banteay Srei is a 10th-century Cambodian temple built of red sandstone to honor the Hindu god Shiva. It's the only major temple at Angkor not built by a monarch, but rather a member of the royal court, Yajnavaraha.
As the temple is smaller than other major temples, 30-60 minutes is enough time to wander the grounds and take it all in. There are many intricate bas relief carvings throughout the temple, so if that's your thing, you may find yourself spending a bit more time.