If you're flying, dont forget to keep a good book for company.
There are direct flights and trains for this journey. Travelling by train is the cheapest way of getting there. New Delhi (Delhi) to Mumbai (Maharashtra) distance is 1428 km and you can get there by train in just over 20 hours and 5 minutes. However, flight tickets come also cheap with fares starting from ₹ 3906.
The weather at Mumbai is varying from "nice" to "hotter than nice".