Delhi has plenty interesting places.
I guess you can spend there a week visiting all sites, but 1 or 2 days can be also enough – depending on your preferences.
What I’d recommend the most are: Red Fort (today’s photo), Humayun’s Tomb and India Gate.
These 3 sites are probably the most famous. In the Fort and Tomb you’ll see mostly tourists, but India Gate is surrounded by locals so that makes it more real.
Of course, you’ll find in Delhi much much more. Visit different monuments, as well as streets and restaurants to taste a bit of everything. 😉