How much do you want to spend? You're taking out for a meal in London by the looks of the other thread, so why not book a nice night in a hotel to go with it and perhaps a show? Failing that if you know she likes Jewellery and shoes and stuff, take her into Selfridges, Harvey Nicholls or Harrods and treat her to something of her choosing (you'll have to be very brave to get it right on your own). Although less personal, you can sell it as the whole "weekend being posh in central London thing" you've got half planned already.