Bus Tours - they are everywhere in the city, and for around £10, will drive you
around the key sites of London, telling you a little of it's history.

They generally last a couple of hours, although your flat fare will let you hop on and
off your bus throughout the day.  (The various operators generally have distinctly coloured
buses - make sure you don't try to hop on another operator's bus, or you will be asked to pay
for another ticket.)

In one day trip you can expect to be taken around most of the main tourist attractions of the inner
city area such as Buckingham Palace, Piccadilly Circus, Trafalgar Square, Harrods, Westminster
Abbey, Parliament House and Big Ben.  All of these areas are relatively close and are within walking
distance of each other if you prefer to see the sites that way.  Wear good walking shoes though!