"Delhi" is actually two cities - Old Delhi, which contains many of the historical/cultural sites and New Delhi, where much of the modern world resides. Both cities, unfortunately, are extremely crowded, polluted and noisy. But the opportunities to take in the culture and history in this city is worth the inconvenience.
During our stay we visited the Red Fort, Jama Masjib (one of the largest mosques in India - 1644-1656), Raj Ghant (home and assasination place of Mahatma Ghandi), Humayun's Tomb (1569-70), Qutub Minar (built 1199), India Gate (War Memorial Arch), Parliament House, Birla Mandir (temple), and Bangla Sahib Gurdwara (largest Sihk house of worship in Delhi. The courtyard of Jama Masjib can seat 25,000 worshipers.