Hey Roshan, you have chosen the best season to spend some good quality time with your loved one. The weather in and around Mumbai is very pleasant during December. I would recommend you to visit romantic and peaceful destinations near Mumbai such as Alibag and Lonavala. Preferably, take holiday suit close to beach or facing the beach. Have a look at the hotels here: https://www.tripoto.com/hotels
Mahabaleshwar , Panchgini : For a hill station honeymoon Alibaug, Murud : For a beach honeymoon Shillim - Hilton (Lavasa) : For a luxurious honeymoon Resorts around Mumbai which are romantic : 1. Fort Jadhavgarh 2. Machan 3. Jalshrushti Happy honeymooning :)