I'm a big fan of random museums. On Gulangyu Island, you can find the Antique Piano Museum. Due to its location, Xiamen was one of the first cities to have a heavily Western influence. It seems that the citizens of Xiamen went wild for pianos.
If you're a musician, you will find this extremely fascinating. Even if you're not, this is a very cool display of both pianos and the history of Xiamen.