Having just returned from a weekend in Paris (dahling) I have a renewed love of cycling. You can get around in half the time, soak up the sunshine AND get some exercise at the same time - perfect for when you've had a few too many macaroons!
And the good news is, London's getting it's own bike hire scheme on July 30 (though they're not quite as chic as the Parisian ones - they have an huge Barclays logo on the side) http://www.tfl.gov.uk/roadusers/cycling/12444.aspx