Lopburi is one of Thailand's oldest cities, a manageable rail trip north of Bangkok and a good antidote to polished heritage towns. Its old centre mixes Khmer towers, Ayutthaya-era royal remains, working streets, and macaques that have made themselves far too comfortable around the ruins.
The headline is Phra Prang Sam Yot, the three-towered Khmer shrine where monkeys loiter on the stonework and nearby wires. Walk a few minutes and the history deepens at Wat Phra Si Rattana Mahathat, King Narai's Palace, and the remains of Ban Wichayen.
Lopburi works well as a long day from Bangkok or Ayutthaya if you start early. Keep food and sunglasses inside your bag near the monkey areas, then give yourself time for the quieter palace grounds and temple ruins, where the city feels less like a stunt and more like a layered old capital.