Ugh, hope I don't regret this. I booked one of their hotel/ticket packages.
Spoke with my sister though, she spent about a week there for the Olympics, about what she would recommend if only having a few days to sightsee. She recommending doing one of the "hop on, hop off" bus tours and also the London Eye. Pay the extra to get to the front of the Eye or you'll be standing in line for hours.