Kunming is called 'the Spring city', which means it is warm all year round, December is the same. But the temp difference is very large this time, sometimes can be as large as 12-20℃. Bring a sweater or thick coat. Pay attention to the weather report before going.
About the attraction sites, I think you can go to the stone forest and the Danchi Lake. You can have a look at Kunming Tours(https://www.travelchinaguide.com/package/kunming.htm) to get some inspiration.