Since you plan to travel in December, I would recommend you visit Spain and Portugal. Some of the popular destinations that you must visit in Spain are Madrid, Barcelona, Seville, Malaga, Granada, Toledo, Cordoba and Ronda. If you wish to opt for a self-drive trip, you can explore the beautiful castles around Northern Spain and the province of La Rioja which is famous for its traditional winery. For exotic honeymoon stays, I suggest the Spanish Paradors which are present throughout the country.
In Portugal, you must visit the capital city of Lisbon, located at the bank of Tagus River and famous for Portuguese historical imprints. You can also visit other cities like Oporto, Guimarães and Sintra which are famous for historical architecture and are also home to some UNESCO World Heritage sites. If you have time, explore the city of Cascais which offers beautiful panoramic views of the adjoining coastline or Estoril which has some excellent casinos and magnificent gardens.