I was in London twice during my stay in the UK. And to be honest, I didn't have any big expectations before as all major cities are similar for me. But I noticed very fast that London is more than just the Tower Bridge and the London Eye. It is a lively city with many different cultures and lots of sights.
If you go for a walk in the bigger squares, you will meet a lot of talented entertainers, musicians, dancers, beatboxers and much more, who simply stand on the street with their instruments, microphones and loudspeakers and entertain people.
When I went for a walk with some friends once, we met fortuitously a lot of people (about 200 people) who all sang together and had fun. This was a very interesting, open event called Massaoke (mass karaoke).
London also has a lot to offer in terms of sights such as Big Ben, Tower of London, Buckingham Palace, London Eye and much more...