Kinmen Island (金门岛) is a curious byproduct of modern history. The Nationalists managed to head off the Red Army's assault in 1949, and it has remained under the administration of the Republic of China ever since. Just a few kilometers from Xiamen, it's the ROC's closest territory to mainland China. During the 1950s and 1960s, half a million bombs rained down on Kinmen and its residents sought shelter in an elaborate network of underground tunnels and shelters.
Today Kinmen is far more sedate. In 2001 it was the first place where direct travel between Taiwan and China was permitted. To this day, it's a major transit hub; domestic flights from the island to Taiwan are far cheaper than their "international" counterparts. Further relaxations since 2008 have brought an influx of mainland tourists. Most of the island today is a quiet, traditional landscape of villages and farms. Jincheng, the principal city, feels like it's out of another age, a warren of small lanes and traditional Fujianese architecture in stark contrast to the modern city of Xiamen. Several Cold War fortifications are open to visitors. A good way to explore the island is by bike, which are available for free rental.
Ferries to Kinmen cost roughly 160 RMB and leave every hour from the International Cruise Center on Dongdu Road (厦门国际邮轮中心东渡路).