Humayun's Tomb in south Delhi, near Hazrat Nizamuddin station, is one of Delhi's three UNESCO World Heritage Sites. Open daily from sunrise to sunset, entry is 10/250 rupees Indians/foreigners.
The tomb is in large, immaculately maintained gardens in the Persian Char Bagh (four corners) style that were thoroughly renovated in 2003 with the Aga Khan's help and are consequently probably the best in Delhi. As you enter the complex, the first major structure on your right is the bulbous, octagonal tomb of Iza Khan, a court noble who built it in his own lifetime, some 20 years before Humayun's tomb. As you pass through the first gate, you will glimpse the dome of the tomb and enter a floral path leading to the second (West) gate, which now acts as the entrance to the giant central garden.
The centerpiece is the eponymous tomb of Humayun, the second Mughal emperor. Built starting in 1562, it was the first major Mughal structure in the city and has been described as a predecessor or prototype of Agra's Taj Mahal. The structures are, indeed, stylistically similar, although Humayun's Tomb is built from red sandstone, not white marble, and was built by a wife grieving for her husband, not the other way around. You can climb up to the second level (the stairs on the west side are very steep, those on the south side less so), and on the south side you will find the entrance into the main crypt where Humayun is buried.
Before you leave, be sure to visit the South Gate, the original royal entrance, from where you can get picture-postcard views without too many tourists in the way. In the southeast corner is the Barber's Tomb, also built in the same style. Historians believe that the emperor's favorite barber is buried in this picturesque tomb made of red and grey sandstone.