Not our anniversary, but for Mrs P's 50th I managed to book a long weekend in Amsterdam (including a candlelit dinner canal cruise) without her knowing anything about it. I convinced her 100% we were just going up to London to see a show and stay overnight, but then on the train up to London (as she thought), just before Gatwick I gave her her birthday present and insisted she opened it there and then on the train. She opened it to find I'd wrapped up her passport, just as the train pulled in to Gatwick in time for us to jump off! Got serious brownie points for that one.