If you are in Delhi, the Qutub Minar is a must see place. The edifices within the complex are marvelous and the complex is well maintained and clean.
The Agra Fort is one of important historical landmarks of India located in Agra city. Its magnificent structure is overshadowed by the popularity of Taj Mahal.
If you plan to visit Taj Mahal in near future, this is a must read guide: best time to visit in a day, which gate to enter, what to bring, what not to bring, dress code, is Taj Mahal a kids-friendly place.