We hired a taxi for a day and visited the Taj Mahal and the Agra Fort which are located in the city of Agra.
The Taj Mahal was stunning and beautiful to see. It was built by Emperor Shah Jahan as a mausoleum for his wife and queen Mumtaz Mahal. The Emperor was muslim, so there is a mosque to one side of the mausoleum. For symmetry reasons there are mosques on both sides, but only the one that faces Mecca is a true sanctified mosque. Hamayan's Tomb in Delhi is was built a few hundred years before the Taj Mahal and the Taj reflects some of its influences.
The Agra Fort is from the same time period as the Taj Mahal. The fort, like the Taj Mahal, lies on the Yamuna river, so you get a fantastic view of the Taj from the fort. This fort is very beautiful, well preserved, and the predicessor of the Red Fort in Delhi.