Changsha is a very nice city, see if you can go on the Changsha Eye (dunno what its officially called) which is huge and impressive. It only seems to go in the evenings though which we found out to our disappointment. There's loads to do and see there.
October should be pleasant, even cool if the wind is from the north. The heat there is a drier heat than in the south like Guangzhou for example, October should be very pleasant. In my experience October will be the start of very changeable weather in places like Changsha - if the wind comes from the north it could be cool and rainy, if from the south hot, pack for both conditions!