Both of have never been to the South, so that is a strong contender. We have no idea where exactly to go though. There is so much to see. We do want to fit in a beach, so the Keralan coastline is winning so far.
Even though we are both of Indian origin, and I was there since 6, I am overwhelmed about the complicatedness of it all. How to get there, where to stay, is it safe etc etc? We have no family there nor do we speak the languages. I can only imagine how harrowing it must be for someone with no connections to the country whatsoever, who simply wants to explore without confusion.
I recommend getting in touch with a very reputable travel agent who has experience in creating completely inclusive packages to India for the western traveler. While this may be on the expensive side, nothing beats peace of mind. You can also do a web search and contact Indian travel agencies, and see what they offer.
But firstly, start off with a good book, and I recommend Lonely Planet's 12th edition of 'India'. It is a gloriously detailed and enjoyable read of 1236 pages, and you will feel a lot better knowing what this book tells you. Borders stores it for $30, but you can get it for $20 at Amazon.