Itmad-ud-daulah has a special place in the chronicles of both history as well as architecture. This is precisely because Itmad ud Daula is the very first tomb in India that is entirely made out of Marble. This is actually a mausoleum that overlooks the River Yamuna and is a tomb of Mir Ghiyas Beg, a minister in the court of Shah Jahan.
This monument is much older than Taj, has amazing detail, and isn't a very busy place. You can spend time, just taking in all the detail and the grounds. Make sure to take your shoes off before entering the tomb.