Phra Prang Sam Yot is Lopburi's signature sight, a compact Khmer-style sanctuary with three linked towers in Bayon style. It would be worth a stop for the architecture alone, but the resident macaques have turned it into the city's strangest theatre.
Treat the monkeys as wild animals, not props. They grab food, bottles, glasses, and loose bags, and the visit is much calmer if you arrive with nothing visible in your hands.