I was in Guilin a few weeks ago and it was still hot ! (shorts & tshirt weather, day & evening) If you're planning to visit the Longji Rice Terraces though, you need to decide what you want to see - the beginning of October the Rice Terraces turn golden - September they're still beautifully green. Yangshuo would also have to be on your list - and I'm sure any time in Yangshuo would be fabulous - though bamboo rafting is, of course, nicer on a pleasantly warm day ... I can only judge on the time of year I was there (end of September) and I loved Guilin, Yanghuo and Longji ! (also don't forget to include XingPing and Daxu - easily accessible by local bus) This is a beautiful part of the world - make sure you allow enough time to see all the wonderful scenery!