Elaine, I did this as a day tour when I was in Prague back in summer 2011. I remember there were two or three different tour companies that offered various day tours both in Prague and around the country. They had booths or storefronts around downtown Prague. I took a few such tours, generally signing up the day before, and all were very good, a lot depending on the personality of the individual guides. I thought each tour was reasonably priced. The tour to CK was pretty much all day. Included a tour of the castle and then at least a couple of hours free time, including time for lunch. One of the things I enjoyed most was the drive across so much of the countryside, observing farms, towns and the seeming prosperity, including various "big box stores" with crowded parking lots. Pretty sure the bus made a brief "pit stop" each way.