Phanom Rung Historical Park

Khmer architecture in Thailand doesn't get more complete than this. The temple sits on top of an extinct volcano, built between the 10th and 13th centuries as a stone model of Mount Meru, the sacred mountain of Hindu cosmology, and the layout does its job. You climb the ceremonial stairway, pass through the gopura gateways, and by the time you reach the main prang you've gained enough height to see the countryside spread out flat in every direction. The pink sandstone and laterite are the draw, along with lintels carved with mythological scenes that have survived far better than they had any right to.

Four times a year at sunrise, the sun lines up and shines straight through all fifteen doorways in a single beam. That brings crowds. Any other morning you'll have the place mostly to yourself, which I'd argue is the better trade. Come early regardless, because there's no shade on that stairway and the stone holds heat.