Delhi has been built, destroyed, and then rebuilt many times during the last 1000 years of its history. Apart from being the capital city of many ancient rulers and the modern India, It has always been the cultural heart of the country.
From temples, forts, churches, museums, shrine of Sufi mystics, public parks to modern state-of-the-art structures its a continuum that will amaze you.
Some of the places that you really should not miss are:
1. Nizamuddin Dargah : Hazrat Nizamuddin Dargah is the dargah (mausoleum) of one of the Sufi saints, Khwaja Nizamuddin Auliya (1238 - 1325 CE). Situated in the Hazrat Nizamuddin West area of Delhi.
2. Humayun's tomb : (Maqbaera e Humayun) is the tomb of the Mughal Emperor Humayun in Delhi, India. The tomb was commissioned by Humayun's first wife and chief consort, Empress Bega Begum in 1569-70, and designed by Mirak Mirza Ghiyas, a Persian architect chosen by her.
3. The Red Fort : is a historic fort in the city of Delhi in India. It was the main residence of the emperors of the Mughal dynasty for nearly 200 years, until 1856.
4. Agrasen ki Baoli :designated a protected monument by the Archaeological Survey of India (ASI) under the Ancient Monuments and Archaeological Sites and Remains Act of 1958, is a 60-meter long and 15-meter wide historical step well on Hailey Road near Connaught Place.
5. The Qutub Minar : is a minaret that forms part of the Qutab complex, a UNESCO World Heritage Site in the Mehrauli area of Delhi, India. Qutub Minar is a 73-metre (239.5 feet) tall tapering tower of five floors, with a 14.3 meter (47 feet) base diameter, reducing to 2.7 meters (9 feet) at the peak
These are just few of the attractions, however it would really depend upon how much time you wish to stay in Delhi ,The visit could be really subjective but all would like to say its worth visiting these sites once in life time.