Hi Khushali, Khandala, Lonavala, Panchgani and Matheran are some of the best hill stations near Mumbai, here's a detailed article on hills near Mumbai, go through it for more information
https://www.tripoto.com/trip/best-hill-stations-near-mumbai
These are my favorites in the same order-
1. Malsej Ghats
2. Mahabaleshwar (little far from Mumbai but doable on a weekend)
3. Lavassa
4. Matheran
5. Lonavala or Khandala - havent been to these but kept it low because they are a bit over-crowded