what to do and hows the weather for Jiuzhaigou?

It is the Spring time locally. The thin coat and thin sweaters are necessary. Besides, the sun cream is needed. The temperature difference is so big. The average temperature at that time is around 10 to 20C according to the previous records, but the real situation in this year is hard to tell.
For visiting that place, I do not think you need to do something, because going there is just for sightseeing.