The Red Fort is a historic fort with beautiful architecture. It was the main residence of the emperors of the Mughal dynasty for nearly 200 years.
Jama Masjid is one of the largest mosques in India. The mosque was completed in 1656 AD and has the capacity to hold more than 25,000 devotees at a time.
Lotus Temple is a Bahá'í House of Worship completed in 1986. It is famous for its flowerlike shape and has won numerous architectural awards.
Humayun's Tomb is the tomb of the Mughal Emperor Humayun. The monument was commissioned by Humayun's wife, Hamida Banu Begum, in 1569-70 and designed by Persian architects.
Qutub Minar is a UNESCO World Heritage Site and the tallest minaret in the world made up of bricks. It was built in the 12th century and is a great example of early Indo-Islamic architecture.
If you are interested in history and architecture, you can also visit India Gate, Akshardham Temple, and Raj Ghat. For a side trip, you can take a day tour to Agra to visit the Taj Mahal. To maximize your fun, you can hire a local guide who can provide you with more insight and information about the places you are visiting.