Half-day from Xiamen- Yongding (or Nanjing) and back to Xiamen again

Yongding is much closer to Xiamen. It may be in a rush to visit Yongding. The interval of bus is around an hour to an hour and a half.
If you want to visit the towers in Yongding, the time may not be enough and there are only four buses for the tower directly from Xiamen.
I suggest you to go to the Gulangyu Island, if you plan to make half day trip around that city.