Seeing the Taj Mahal physically was the most memorable and unbelievable feeling compared to seeing it in pictures.
Mumbai ia the gateway to India. It's a must to see how people live in this, the world's most populated city. My best experience in Mumbai was walking around -- left, right, stop, I didn't know what to do it was the most exciting. The people are very friendly, too.
Quick Tips:
A must is to take a photo touching the tip of the Taj Mahal. Don't take a picture with your own personal camera; instead, pay the small fee of 60 Rupees to the guys that take photos -- it's a photo you will never forget.
Best Way To Get Around:
It's best to get to Agra by train. You get to see the whole countryside and it's very scenic.
To get to Mumbai, take the plane from Delhi. Book your tickets from India because it works out much cheaper. The meals are also a good experience in the plane and totally different compared to the Indian food I eat.