Jinan, Qingdao, Weifang or Yantai???

But which city are you departing from? Anyway, geographically here is my recommended route:
JN - Weifang - Qingdao - Yantai
For the first two journeys, bullet trains are reommended to use. From Qingdao to Yantai, you can take an easy travel by bus!

You are right that it will be cold in Dec. in these cities, so it is impossible to swim unless you are good at winter swimming. But you can still have fun on the beach.