If u wanna explore Mumbai in one day, best option is book a ticket for bus in Mumbai darshan. Which will cost you max to max rs300*to 400*u can usually get discounts on the app of various bus booking app also.
If you wanna travel by your own means, you can explore the below places Gateway of India
Elephanta Caves
Colaba Causeway Market-Fashoin Street Chatrapati Shivaji Musuem
Jahangir Art Galary
Victoria Terminus
Nehru science center Nehru planetarium Girgoan chowpaty Taraporewala Aquarium
Marine Drive
Haji Ali
Worli Seaface
Kamla Nehru Park
Siddhivinayaka Temple
Mahalakshmi Temple
Mount Mary Church
Juhu Beach
Sanjay Gandhi National Park
Kanheri Caves
Powai Lake
Global Vipassana Pagoda
Hi Jaydeep, Mumbai has several amazing places to be explored. Tripoto Wanderer Tanya has beautifully captured her few days in Mumbai here: https://goo.gl/GaNfRc You can also explore what travellers of Tripoto community covered in their 2-day in Mumbai here: https://goo.gl/D82fB4