Walk. One more +1 for Southbank, and a walk through the City on the weekend is quite nice - there's a path that runs along the old Roman wall more or less, and then to the Barbican (which can be a hit or not). If they're not big walkers, some bus routes are pretty much tourist buses (23, from Liverpool St Station, through the City, past Bank, St Pauls, Fleet St, Trafalgar Square, Piccadilly, Oxford/Marble Arch). Sunday Markets (Spitalfields, Petty Coat Lane, Brick Lane, and Columbia are all minutes away from each other, and all very different).
Walk. One more +1 for Southbank, and a walk through the City on the weekend is quite nice - there's a path that runs along the old Roman wall more or less, and then to the Barbican (which can be a hit or not). If they're not big walkers, some bus routes are pretty much tourist buses (23, from Liverpool St Station, through the City, past Bank, St Pauls, Fleet St, Trafalgar Square, Piccadilly, Oxford/Marble Arch). Sunday Markets (Spitalfields, Petty Coat Lane, Brick Lane, and Columbia are all minutes away from each other, and all very different).