A spa is a brilliant idea, but I did think that The Sanctuary in Covent Garden was women only?
A weekend away in the countryside, in a hotel with a spa and some treatments thrown in, should please her. If you make it near a town/city with good shops, then she will probably love you forever. Bath, Oxford, Winchester, Norwich are good ideas.
Where abouts do you live? Also, don't travel too far. A weekend away can be spoilt by too long a drive, and getting caught up in traffic. Make it extra special by arranging for some flowers in the room, or maybe chocs and champagne.