Shaheed Bhagat Singh Marg on Colaba Causeway has a street market you could explore. If you can afford it, have a coffee at Shamyana coffee shop or Star bucks at the Taj Mahal hotel, OR explore the shopping arcade at the Taj just to get a feel of the grand edifice said to have been build by Jamsetji Tata, as an answer to the British when he was turned away from entering one of their hotels.

For lunch and dining, there are places in all price ranges you could try out, like the Indigo restaurant serving fine European food, and more reasonably Leopold cafe, and Bademiya, famous for their Sheekh Kababs, in the Colaba environs.

Colaba- View from the backwaters 

Dr. Chhatrapati Shivaji Terminus formerly Victoria Terminus, at the north end of the Fort is worth a visit. Catch the walking tour of this  magnificently carved municipal train station, and the lit up CST at night is a sight not to be missed. 

Nearby Water field road has some of the best dining options. Moti Mahal for Kebabas, Thai Ban for Thai food, Dosa diner for dosas, China Gate for Chinese are some of the reasonable options.

A boat ride from Gateway of India will take you to the tiny island of Elephanta in the middle of Mumbai harbour. Be warned. After the boat ride, almost an hours walk including climbing the uneven rock steps to reach the caves is quite strenuous. The train that is supposed to take you half way is mostly out of order. The place gets quite filthy on a holiday with the crowds throwing plastic covers and eatables around. It is a pity that Maharashtra tourism is not taking any effort to maintain the renowned world heritage site, with a series of cave temples and fine rock carvings.