Planning Your Time
It's a good idea to take an express train to Agra to avoid Delhi traffic. To see the Taj and related sights without being completely rushed, plan on spending at least one night in Agra.
Most people visit Khajuraho in conjunction with Varanasi: the best idea is to fly here either on your way to or from Varanasi. Plan to spend two days exploring the Chandela temples.
Varanasi requires at least two days, including half a day for Sarnath. Again, the best option is to fly from Delhi, perhaps stopping in Khajuraho.