Hello Sultan,
What a wonderful time to travel. No lines at the museums and monuments, wonderful roasted chestnuts on the sidewalks of Madrid. Lovely December skies.
First, you must decide what your pace is going to be. Your itinirary will depend on the number of cities you would like to visit. You have about 16 days to travel, you don't count the 18th nor the 4th. Remember that the evening of the 24th, the 25th, the 31st of December and the 1st of January are party days in Spain.
Of course, you could decide to spend all your time in Madrid, there is plenty to see and do in Madrid and its environs. The Prado museum deserves a whole day! Near Madrid you have the unforgettable cities of Toledo, Segovia, Avila, Alcala de Henares, and El Escorial.
Madrid is a centrally located capital, so if you wish to travel afield you can accommodate a trip to Sevilla, Granada and Cordoba. Or travel northwest and visit Salamanca, Leon and Santiago de Compostelo. You can travel northeast to Bilbao and Barcelona. Or west to Valencia and Murcia.
When I travel I choose two to three hub cities, and then I plan day tours to nearby cities. I usually haunt Barnes and Noble bookstores and look over the newer travel books for ideas.
Good luck with your research and eventual trip,
Carmen
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The world is a book, and those who do not travel read only a page.
--St. Augustine (354-430)