The Taj Hotel, the Gateway of India, the Air India building. Walking through South Mumbai and the Marine drive is a pretty soothing experience, once you experience the chaos and congestion of the other parts of the town.
The Hiranandani Gardens have some interesting buildings, restaurants and places for shopping.
the greenery, the water and the small hills,
After that, if you are a Bollywood fan, you must visit Bandra. In Bandra, Bandstand and Carter Road are really beautiful and as bonus you might spot huge Bollywood stars like Salman Khan, SRK etc.