I proposed to my wife at the top of the Sacré Coeur in Paris, following up the proposal with a lovely lunch at Les Ambassadeurs. The proposal was a surprise, even though the trip itself involved no real subterfuge (other than secretly asking her father for permission to marry her in advance of the trip). It was at the back-end of a planned trip to Amsterdam, London, and Paris. She knew we were going on the trip beforehand. When we got to the top of the Sacré Coeur, we nearly had the place to ourselves, so the moment was rather intimate. There were two things that were less than ideal -- it was a rainy day, so my wife was having what she considered to be a bad hair day, and her climbing a bunch of stairs in very high heels wasn't the easiest thing in the world -- but the proposal and the ring more than made amends.

To pwb2103, I might suggest that you incorporate your girlfriend into your trip planning, that you dispense with the subterfuge for getting her to Spain, that you carefully pick a memorable spot for your proposal, and that you hold off on making your proposal until near the end of the trip so that whatever suspicions she may have had about the trip serving as a platform for you proposing probably will have dissipated by the time you finally make your move.