Believe it or not, you can book your own city tour and guide yourself through it. Many locations offer city tours that are simply rented equipment that will take you around the town and tell you great bits of information that you need and want to learn.
Some city tours can take you into locations that aren't open to the public. For example, if you visit the United Kingdom or Ireland, you will be able to visit the various castles and maybe even get inside to meet some pretty great people and see the grounds.