Old Delhi: Delhi is split into two parts, Old and New. Old Delhi is the more charismatic of the two, with the narrow streets and market vendors that hearken back to an earlier time. Old Delhi is also home to the UNESCO World Heritage Site, Red Fort (Lal Quila). This was the main home for the Mughals (the wealthy Muslim dynasty that not only ruled India for more than 300 years, but also coined the modern term "mogul") that also served as the seat of government between 1639 and 1857. There's an English sound-and-light show held here most evenings; timing depends on season.
Gandhi: No figure influenced modern Indian society -- and, indeed, the world -- as much as Mahatma Gandhi. The icon of civil disobedience is revered in India and there are two sites where you can learn more about him. Gandhi Smriti is the home where Gandhi lived the last 144 days before his assassination; the 1948 shooting also took place here. Take your shoes off and stand before the Martyr's Column, where Gandhi had been praying before he was killed. Besides being a peaceful respite from the hubbub of Delhi, the National Gandhi Museum has many photographs and quotes from his life; spend as much or as little time as you need. Watch out for the monkeys roaming the grounds.
Humayun's Tomb: A precursor to the Taj Mahal (which is indubitably the highlight of your Golden Triangle tour), this garden tomb built by Mughals is also a UNESCO World Heritage Site. The red sandstone structure dates back to 1570 and -- as a counter to the Taj Mahal -- was built by an Empress, Bega Begum, to commemorate her husband, the Mughal Dyansty's second Emperor. Like many of Delhi's tourist sites, the other visitors are almost as interesting as the site themselves; you'll see women in colorful saris, scarves and abayas. (Mathura Road, opposite Dargah Nizamuddin.)
Qutub Minar: The third UNESCO site in Delhi -- Qutub Minar -- also draws visitors from around the country in colorful dress; the complex is also a frequent backdrop in Bollywood movies (and you'll see young Indians reenacting favorite scenes). While the surrounding monuments should be inspected for their graceful calligraphy and carving, it's the towering minaret that dominates pictures. Construction of the red sandstone tower began in 1200 and it's still solid, although it does noticeably lean (people are no longer allowed to go inside and climb after children were crushed in a 1981 stampede). Also take a look at the Iron Pillar of Delhi, an otherwise nondescript piece of metal that has resisted corrosion for centuries.
The Food: To Westerners used to the general term "Indian food," Delhi will be a culinary delight (just be careful what and where you eat; see "Watch Out For" section above). The regional dishes here are considered North Indian, and come heavily spiced. Popular dishes include biryanis, kebabs, koftas and paranthas (stuffed flatbreads). While the luxury hotels have outstanding Indian offerings, you can also visit a market with sit-down restaurants such as Khan Market to try something new (we don't recommend street food unless you are touring with a knowledgeable guide; even then, don't overdo it right away, as your gut might not be used to different spices and microbes).