Progol : First of all I would like to thank, it feels great when somebody admire India. Well its really a huge post but not boring actually. As you mentioned the places here Kerala, Tamil Nadu and Mumbai. In my opinion among these 3 Kerala is one of the best places in whole India. Must See : Kerala backwaters, experience house boats and tree houses there and if you want some peace and relax then Kerala is also very famous for its Spa and Ayurveda. I will recommend you to try once(if your pocket allows). Yeah Tamil Nadu and Mumbai also good places but totally different. In future, if you want to travel north India then I will give you much more better information cause I personally like mountains, rivers, lakes, forests and landscapes. Anyways Thanks :)