From the meeting point, start the 3-hour walking tour by heading through Green Park to Buckingham Palace, admiring the well-manicured trees and flowers along the way. At the palace, watch the world-famous Changing of the Guard Ceremony.   Afterward, see other iconic London sights such as Downing Street, Westminster Abbey, Big Ben, the London Eye, Parliament Square, and the Houses of Parliament.   Next, head to the O2 Arena and get ready for a guided climb across the arena roof for scenic views of London's landmarks. Lastly, take the cable car for an easy ride across the River Thames, allowing for further amazing views of the London skyline from up high.