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 :)