After visiting Jantar Mantar, the real fun begin.
We change our plan and use Metro train instead of rental car.
We live near Connaught Place, so nearest Metro Station is Rajiv Station. Our destination is Qutub Minar which is 5/6 stations far from our place.
For those who might be interested, here is a bit of info on the Delhi Metro System...
The 213 km network (and rapidly growing) serves the city of Delhi and its satellite cities i.e Faridabad, Gurgaon, Ghaziabad and Noida through 160 stations with a mix of underground and elevated lines. Phases 3 and 4 will see the addition of 28 more underground stations and over 41 km of track. The Delhi Airport Express is a 23 km high speed line connecting IGI airport to the city center. (Thanks to @Bombaywalla )
The Metro map
