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.
Scaling the Eiffel Tower has a price, but the view from below is spectacular in its own right, and a perfect backdrop to a leisurely picnic in the surrounding grassy area. Be sure to visit the tower at night when it sparkles each hour with an awe-inspiring ten-minute display of 20,000 glittering white lights.