Dongguan sits between Guangzhou and Shenzhen, and for many travellers that is exactly the problem: it is easy to treat the city as the industrial blur outside the train window. That misses the oddness of the place. Dongguan is a huge manufacturing city, but it also contains Humen, where Lin Zexu's destruction of imported opium helped trigger the First Opium War.
The strongest day is in Humen. The Lin Zexu Memorial Museum and nearby Weiyuan Fort put the Pearl River into the story of Qing decline, British gunboats, and China's century of humiliation. The fortifications are not romantic ruins so much as hard political history in brick and stone.
Central Dongguan is softer. Keyuan Garden is one of Guangdong's classic Lingnan gardens, compact and clever, with pavilions, water, and corridors folded into a small urban site. Qifeng Park gives the city a green ridge and a useful viewpoint, especially if you need a break from highways and malls.
Dongguan is not a first China trip essential. But if you are already in the Pearl River Delta and want a day that explains why this region mattered before electronics factories and high-speed rail, Humen makes the case.