See if you can book a carriage on the London Eye, prolly super cheesy, but you know, it's the ultimate day for cheesyness.
Then for the night, you could take advantage of the many Hotels in the area.
Good luck tho, cos it prolly is kinda late for booking it all.
the Sunday can then be left for her to go shopping in Harvey Nichols or somewhere, as a cherry on top sort of thing.