Depends if you want to spend much money, personally I'd suggest a walk, get a train to Westminster, then start will be the Houses of Parliment, taking in Big Ben (yes I know thats not it's real name), then over west minster bridge, along the south bank past the wheel, past the NT taking in a bit of culture, then on towards the new London Assembly. Then have a look round boro market. Then get a tube to Charing Cross and come out in to Trafalga Squard, the national gallery and then a short walk to either covent garden or Lecister square.
Job done, all free, a bit of fresh air and plenty of touristy things.