The Batobus is a great way to see the city. It has 8 stations along the river, all located at major sights, and you can buy a one day pass for 16€ and hop on and off as you please all day.
Although you are limited to the sights along the river, this is where many of the iconic venues are, and you can easily see the Louvre, Musée d’Orsay, Notre Dame, the Eiffel Tower and more using the Batobus. Just buy your pass and wait at one of the stations as you would for a bus. It’s a great way to spend a day visiting, and you can save the sights further afield for another day.
Check out their website for more information. Batobus