Our Delhi co-workers brought us to visit Qutab Minar (there seem to be many different accepted spellings of this place!) one afternoon as a break from all the training we were doing. It was worth the short trip from our offices -- but it's about a 45-minute taxi ride from New Delhi.
This ancient complex was built in the 12th century and includes ruins of India's very first mosque, and a series of tombs and mausoleums. It reminded me of places I'd been to in Greece and Rome -- almost like a smaller Forum, but obviously not as old.
The incredibly detailed lattice-work and delicate screens carved of red sandstone are just beautiful, decorated with the writing of the Koran in calligraphy, and designs of the lotus flower. I would plan to spend at least a few hours exploring the grounds here, and even on a hot day there's plenty of shade provided by the buildings and the trees sprinkled throughout the grounds.