回複：九月中去黃石會不會太遲了？一直沒時間去度假，

It will not be as cold as getting snow, but in the morning there could be frost. Mid-late September is the time for the leaves to change color and you probably have better chance meeting a bear. Comparing with the number of camp sites the hotel facilities within the park are limited and all operated by the same company; you should be able to get reservations by checking this site

http://www.travelyellowstone.com/

Of course you should be able to get better deal in Jackson (Jackson itself is quite an interesting town) I suggest to stay at least one night in the park to explore the better photo opportunities in the early morning and late afternoon (and also to avoid the tour bus gangs).