Hop-on, Hop-off Buses
Hop-On, Hop-Off bus tours, also often known as loop tours, provide a quick and convenient way to get an overview of a city. They are frequently used by visitors on their first day or two in a new city as it can quickly give you an overview of how the area is laid out, while a tour guide provides a history and interesting facts of the city as well.
After seeing most of the major sites via the hop-on, hop-off bus, I can honestly say that it can help you to determine your itinerary for the rest of your stay (as it can often help you decide which area of the city you might want to visit again on your own and which sites you might like to see in more detail).
Hop-on, Hop-Off tours are also used by visitors who only have a very short amount of time in an area or specific city, so if you ever want to get a real feel for a city (and quickly), the loop tour enables you to covera lot of ground in a short period of time and also allows you to experience most of the major attractions of the city you are in.