I only had a limited time in Delhi, so I got a coach tour arranged by one of the local hotels. It was much cheaper than getting a reliable driver and car. A few have stopped offering them due to problems with bad drivers (which we had!), bad busses and generally bad service. I think that we ended up getting it from the Oberoi in Delhi, but don't know for sure. It might have been one of the Taj hotels.
We also got a tour of the Agra fort and a couple of other sites, breakfast (weak, though good curried potato thing), lunch (5* buffet) and dinner (even weaker than breakfast!)
I almost didn't take the trip due to the length and the fact that I was a bit under the weather, but it was totally worth the very long day.
Enjoy,
Dr. PITUK