There are loads of companies offering "Bateaux-mouches" tours on the Seine. I'm a particular fan of having dinner on one of the boats. Of all the companies, I recommend "Bateaux Parisiens" for lunch/dinner cruises and "Canauxrama" for its more adventurous trips outside the city.
Going at night is as impressive as during the daytime, as both banks of the Seine are brilliantly lit. Notre Dame, l'Hotel de Ville and the Eiffel Tower (amongst others) are famous sights all in close proximity to the river and it's a great way to orientate yourself if you're new to Paris