If you've only got a day I might croos to the south side at Westminster, that's where the London Eye is, it's also got the South Bank Centre, which hosts all sorts of events, this is not far from Tate Modern, you can carry on walking down to Tower Bridge via Bermondsey, have lunch at one three different Conran restaurants go to the Design Museum, cross over for the Tower, and walk back through the city, possibly via Whitechapel or St Katherines Dock (I recommend the Mala Indian restaurant).