Berlin is a great city and as a business traveller with one day to tour the city, the recommendation is those busses that are "drop on, drop off." The price is between 13 to 20 euros all day, that is very cheap considering the taxi prices in Europe. They start at 10 am and end at 5 pm, and they stop at every important place you must see in the city, including a well known shopping mall and Kudam Ave. where all the important stores are (so you can buy your gifts in a short time, everything is together).

 

If you have only one day to see the city, this is a good choice.