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.
Pont des Arts Love Locks pedestrian bridge in Paris, walking distance from the Louvre. Write you and your loves name on the lock and throw the key in the river. Would love to do this some day when I go to Paris!