A visit to the Panthéon is a bit of a mini-course in French history. There are incredible mural style paintings of great moments like the crowning of Clovis, the first Christian king of France, and the saving of Paris from starvation by St. Genevieve in the 5th century; lots of monuments to French military heros; and a crypt full of great men (and yes they're mostly men) like Rousseau, Voltaire, Dumas, Zola, and Louis Braille.
There's even a re-creation of Foucault's pendulum which proves that the earth actually turns. You may have seen such a pendulum elsewhere (as I did in the Smithsonian in Washington) but you should know that this is the site where M. Foucault actually did the experiment in the first place.
All very interesting but the only thing that motivated me to pull out my camera was this:
Fossilized remains of sea creatures from a long ago France, immortalized on the front steps.