Go to the South bus terminal (you can take the C71 bus from Kunming Train Station). You can take a bus from the South Bus Station to Chengjiang. From Chengjiang bus terminal you can take a city bus (the #2) directly to the lake. There are some places to sleep on the lake, but I'm not sure about the prices. Food seems pretty cheap here and the people are proud of their fish dishes. You can swim in the lake, rent a paddle boat, and hang around. Its a nice setting for anything relaxing. Have fun!