There are many places which you can plan to visit in September and it completely depends on your choice that what kind of a trip you're planning for. Late in September is good for mountains of Uttarakhand, Meghalaya etc. For waterfront, Goa is an all time fav for tourists and September end is equally enjoyable. Also, for only sightseeing, there are states like Rajasthan, Gujrat, Madhya Pradesh where you can plenty of options. If history lover, Hampi, Badami, Pattadakkal, Mahabalipuram etc are paradise for photographers and history buffs.