Tate Modern/Millenium Bridge/Globe Theatre/St. Pauls especially on a day like this is a lovely way to spent the afternoon
Trafalgar Square http://www.oneandother.co.uk/, National Gallery, and the Portrait Gallery
Andy and I did this on the last day of my visit...the day before, we crossed the river and just strolled up and down the South Bank, up Westminster Bridge...took tons of pics and saw so many new things!
If she's never been to England before, everything will be a new experience for her. I was flabbergasted every time we got into the Underground! Maybe I'm just easy to entertain, lol...
We also visited Avebury and the East Kennet Long Barrow...it's about 90 miles west of London, so not too bad for a day trip, and really a very interesting and pleasant walk around some gorgeous countryside, if you're looking for something outside of the hustle and bustle of London.
The British Museum is phenomenal as well, and free to boot! If you're planning on going on the Eye, book your tickets in advance....on a nice Saturday night, the line stretched all the way to Leicester!
More than anything, make sure she brings lots of extra batteries for her cam...I ran out on Westminster Bridge, and I'd forgotten to bring the extras I lugged across the Atlantic...thankfully, Andy had some spares on him!
Enjoy your time...that goodbye at the airport always comes quicker than we expect!