In Xining Bus Station in Jianguo Rd which is only 200 meter from the ralway station, there are many buses from the morning to aurrounding towns near the lake. Seems there is no direct bus. So it will be a little trouble while getting back. Generally, one day there is enough. It is expensive to renting a van there, bu perhaps, you can rent one with others. To join a one day tour is most convenient and cheap, but I assume that you will not likely to. Well, if you renting a van, you can take a look at other sight on the way to the lake. For example, Sun and Moon Hill, prince Wencheng Temple. Good luck.