South Indian is best option if u seeking both romantic destination with temples. U can start from Bangalore & cover Bandipur forest, masinagudi tiger reserve, Ooty Nd Coonoor. If you have more days in hand you can travel to Coorg also.
Uttarakhand : you can cover rishikesh, mussoorie, chopta, auli. In Chopta you can cover the tungnath temple. You can also visit Haridwar on the way to rishikesh and cover the famous Temple at Har Ki Pauri. You can also cover The famous temple at Triveni Ghat in rishikesh and attend the Ganga maha arti in the evening. Himachal: you can cover shimla, manali, dalhousie, dharmashala. You can visit the famous Hadimba temple in manali. You can visit the very famous Hanuman temple that is Jhakkhu Temple in Shimla. you can go for Himachal if you want to eitness a lot of snow. And you can go for Uttarakhand if you prefer less snow but lot of beauty along with temples.
Go first kolkata, visit Kali Ghat, dakhineswar temple..Then plan for Mandermani or Bokkhali Beach resort for 1-2 days, then you go for Darjeeling hills by train or flight from kolkata..December month has very romantic weather in Darjeeling.Moreover kolkata weather is good that time..Enjoy ur holiday..