The wide boulevards are superb as you can walk along this and never feel crowded no matter how many people are about.
We spent eight days in Paris and saw most of the major tourist sites - here is a photo of me with the Seine in the background.
It was the most relaxing place, particularly in the morning as nothing opened before ten. Paul and I were out every morning by 8 and had it to ourselves. We then checked out whatever we wanted for the day and then wandered home. As our hotel was in the 5th arrondissement it was a walkable distance to everywhere,we probably walked about 10kms each day, which helped to counter the baguettes and cakes we ate daily.