London is a mega-city, one of only two cities rated as Alpha++ in terms of infrastructure (the other being New York). I’ve visited the UK’s capital way back in winter 1996, then again recently in fall 2012. Lots has changed over the year of course, except for the historic areas obviously.
So much to see and do in London — visiting the famous Leicester Square, Piccadilly Circus, Hyde Park, Bond St, Mayfair, Trafalgar, British Museum, London Bridge/Tower, Buckingham Palace, Big Ben… just to name of few popular sights. More recent attractions include the O2 Dome, London Eye, the Shard, Emirates AirLine (cable car) and the new Canary Wharf area. The Underground (or Tube) itself is an attraction!
London is quite an expensive and crowded city however, and a longer visit could wear you out.