If you have a technical or historical bent, you'll probably enjoy an hour or so learning about the Parisian sewer system. It's one of the earliest efforts in humans dealing with waste removal and, of course, one of the major steps forward in health standards for large cities. It's pretty amazing to see how the sewers are maintained and cleaned. The museum is all underground and despite the subject matter is not an assault on your senses. We took our teen age daughters there and even they showed at least some interest. This is a nice change of pace from the other sites in Paris.