I will be passing through Kunming and will be there for 8 hours (12-8 PM). What places can I visit?

Yes, you can store your luggage in the airport. Location is:
1. Beside the staff channel, 3F;
2. International Arrival Exit, West side of -1F.

8 hours are not that long. But it is still enough for you to have a quick visit to city center. From airport, you can take shuttle bus line 3 to Expo Garden, then walk to Shibo Jixin Garden to take bus A1 to Jinmafang, where there are some traditional Chinese architectures. Also, there are many restaurants there, where you can enjoy a delicious local meal. It takes about 80 minutes for a sinlge trip and remember to go back to airport on time.