Considerating the weather in December, I think perhaps Guilin, Yangshuo, Kunming, Hainan area are better destinations. Here are some group China tours(https://www.travelchinaguide.com/group/) which can give you more choices to decide where to go. For how many days you have or would you prefer travel alone, these are also decide your travel plan. If you want to travel alone, I think you should go avoiding small places. The, Guangzhou, Shenzhen, even Macau and Hong Kong are nice choices.