Delhi is usually seen as a stopover to explore North India at large: Rajasthan, Ladakh, or of course Agra. Its points of interest are way underestimated! Why not enjoying your stopover to discover the city's incredible monuments hidden in the middle of a crazy traffic, and a quite dusty environment? And by doing it, your vision of the capitol will completely change! You will feel traveling back in times to the era of Moghul Emperors in which "Dilli" was the powerful respected vibrant jewel of the subcontinent.
It is biased to give any listing of the city's monuments, but I prefer to see it as a personal approach based out of my experiences. Here are some beauties not to be missed: the Red Fort and the Old Delhi mosque Jama Masjid, Humayun's Tumb, Qutub Minar,...