I will suggest you to visit hilly area's in north region like Shimla, Kullu & Manali or North East region both of them will have pleasent atmosphere while other regions will have either have a high temperature like as in Rajasthan or heavy rains like as in southern states. thanks
Mumbai is awesome if you love rainy season.....you can go to Lonavala, Khandala and Mahabaleshwar. Transportation is cheap, you can visit these places easily and enjoy a lot.
You can enjoy treks at these places also.