In a tiny cheap room, I woke up at 5am to go straight to the Taj Mahal less than 500m from my hotel, the gate are supposed to open at 6am. The Taj is grandiose, beautiful, extravagant and crazy: it is a mausoleum for the wife of the Emperor Shah Jahan when she died giving birth on their 14th child in 1631. It is probably the world most famous white marble building. White marble is actually still pretty common in India, especially around Jaipur (seen later). That said, if the calligraphy on the Taj is impressive, the rest of the decoration is not as detailed as many other stone buildings.
Next in the same marathon day, I visited the Agra Fort, quite similar to the Red Fort in Delhi but actually bigger, it was the fort of the Mughal emperors, i.e. with a lot of Islamic influence. You can enjoy the marble part build under Shah Jahan reign.
As recommended by Abishek, I went to Sikandra to see the next big Mughal structure, namely the tomb of the greatest emperor Akbar. This tomb is also a little similar to the Humayun’s tomb in Delhi, the antelope in the park around gave however a better ambiance! And the sunset was beautiful! Going there with local bus was quite easy (8km from Agra), going back was more tricky since I did not find a bus going back to the Agra Fort bus stand. So I ended up using one local bus and two shared rickshaw. Some adventure again!
This Akbar emperor built a whole new city to make it is capital, but he was the only one who stayed there. Thus, Fatehpur Sikri is like a ghost town. Going there is straight forward with the local bus (1h, 30 Rs). Going there through the bazaar to the Mosque and then this Mughal city was a little crowded, the best part of it was again to get out of the crowd and go behind the touristic area to see maybe not so beautiful construction but that you can better appreciate, being alone on the site.