If you have some time on hand, can try Elephanta caves, ferry ride from Gateway of India. Marine drive (Queens necklace) is good view at night. If going to SGNP, there are also caves bit further inside, called Kanheri caves which is worth going to. Sunset at Bandstand is good. You might also want to visit Mahalakshmi temple and Haji Ali, again worth visiting and is in proper city, not far away from airport. Hope this helps. Enjoy your time.