No trip to Paris is complete until you make one visit to the Eiffel Tower. I've never actually been to the top of it (lines are always too long), but it makes for a remarkable backdrop to any photo. The Eiffel Tower is the tallest building in Paris and can be seen from most parts of the city. It was under construction when I visited (building a new elevator system in the middle of it)... kind of interested to see what it looks like when they are finished.
Recommend getting a nice viewing area of the Eiffel Tower as the sun goes down... puts on a nice light show.