The "Love Lock" bridge in Hamburg Germany is just cool. You buy a lock, have the local jewely etch your names on it, lock it onto the wire of the bridge and together toss the key in the water. I was a neat experience.
Love Lock Bridge, Paris---I was just there, and what a stunning view from the bridge! The lock idea is really interesting too, and the story that goes with them :) Even if it's not good for the bridge...:p