If you can not visit Agra to see Tajmahal, at least visit Humayun's Tomb in Delhi. In fact the design concept of the world famous Taj Mahal was derived from the structure of Humayun's tomb. Both are tombs, but Humayun's Tomb was built 80 years before Taj mahal. Therefore historically also it is good to visit monuments of Delhi, including Humayun's Tomb, before you visit Agra.
As you enter the main gate, you see a well maintained green garden surrounding the brick-red structure of Humayun's Tomb, which adds to the beauty of the monument. There are other tombs and structures in the campus which are damaged with time but tell the history of that era.