Of all things to do in Paris, this experience is the most serene. You could take a one-hour long paid tour on the river and witness some of the breath-taking Paris tourist attractions far from the crowd and bustle of the tourists. You could also decide to ditch the popular monuments and decide to take a tour through one of the hundreds of canals and waterways in the city.
Tip: Do remember to dress warmly when you visit the Seine, as it could get quite cold, especially during the evenings.
Alternatively, you could decide to stay away from the extra touristy places and take one of the many bus tours through the city. This way you can find your own things to do in Paris, and not have to deal with large crowds.