In the morning, enjoy the scenic view of the Arabian sea from the iconic Gateway of India. Take a boat ride to Elephanta Caves in the afternoon and watch the sunset at Marine Drive in the evening.
Learn about astronomy and space exploration at the Nehru Planetarium in the morning. Have lunch at a nearby restaurant and visit the nearby Haji Ali Dargah in the afternoon. Spend the evening at Juhu Beach.
Spend the day at Sanjay Gandhi National Park exploring the wildlife and nature trails. Have lunch at a nearby restaurant and visit the Kanheri Caves in the afternoon.
Spend the day at KidZania, an indoor theme park where kids can role-play as professionals. Have lunch at a nearby restaurant and visit the nearby Powai Lake in the afternoon.
Take a day trip to the Elephanta Caves, a UNESCO World Heritage Site. Have lunch at a nearby restaurant and visit the nearby Global Vipassana Pagoda in the afternoon.
In the morning, visit the historic Chhatrapati Shivaji Terminus railway station. Have lunch at a nearby restaurant and visit the nearby Crawford Market in the afternoon. Spend the evening at the Worli Sea Face.
Visit the Siddhivinayak Temple in the morning, Mumbai's most famous temple dedicated to Lord Ganesha. Have lunch at a nearby restaurant and spend the afternoon at the nearby Mahalaxmi Racecourse. In the evening, enjoy the sunset at Bandstand Promenade.
You should also consider visiting the Nehru Science Centre, the Chhatrapati Shivaji Maharaj Vastu Sangrahalaya museum, and the Chor Bazaar for shopping. You can take a side trip to the nearby hill stations of Matheran or Lonavala. To maximize your fun, try local street food like vada pav and pav bhaji.