It is very difficult to pick one single thing as almost everything we saw and did was new and exciting.
The Taj Mahal at sunrise was incredibly beautiful but the Amber Fort was equally impressive.
Sunset on the Ganges at Varanassi was a moving experience, as was seeing the funeral pyres at Kathmandu.
A visit to a primary school, seeing the bright and polite young children working hard to improve their lot, was a bonus.
Be prepared for an experience, rather than a restful holiday, but take advantage of everything on offer as far as sights and trips go, as well as the food.
Don't rely on having any clothing laundered while away; far better to travel with enough lightweight clothes to survive the trip.
Make sure you have plenty of batteries and memory cards (or even film) for your cameras - there is just so much to record for future enjoyment.
The large, three round pin, electrical adaptors which some websites recommend, do not fit in all hotels. The smaller, two round pin, adaptors work almost everywhere.