Rue Soufflot, 75005 Paris
Métro: Cardinal Lemoine (see below)
The Panthéon houses the tombs of lots of France's most famous and most dead people, including the likes of Voltaire, Victor Hugo, Louis Pasteur, Marie Curie, Émile Zola, and Louis Braille. (Have you ever seen Louis Braille's tomb? Haha. What? Too soon?) The interior of the building includes an enormous, breathtaking, basilica-like space on the ground level, with high domes and gigantic paintings, and then the somber rooms of the mausoleum are down on the lower level.
Incidentally, they have a Panthéon in Rome too; I think they're all the same, like Starbucks. (Haha, okay the Panthéon in Paris was modeled after the one in Rome, which was built in the second century.)
Getting to the Panthéon can be a tiny bit of a hassle, because there's no Métro station that's really close by although I should mention, in Paris there's never NOT a Métro station that's relatively close by, so I'm talking about maybe a 15-minute walk here. The actual closest Métro station is Cardinal Lemoine, but it's on line 10 which isn't a very practical line, and it puts you behind the Panthéon so you're approaching it from the wrong side, and I think a nice part of the experience is approaching it from the front if you can, walking down Rue Soufflot and seeing it at the end of the street the whole time. So, a few other options:
There's an RER (regional rail) station, called Luxembourg (line B), that's in a pretty ideal place; right near the end of Rue Soufflot. But I personally find that taking the RER for short trips within Paris can be kind of annoying and impractical. So if you're up for a short walk, I think your best bet is taking the Métro to Odéon (lines 4 and 10) and hoofing it. This option will take you on a leisurely stroll through small streets lined with little art galleries and old book stores, past the gorgeous Luxembourg Gardens, and puts you right at the far end of Rue Soufflot so you can see the Panthéon in the (near-)distance as you make your way toward its gates. So it's a perfect opportunity to work in your visit to the Luxembourg Gardens on the same afternoon.
The path from Métro Odéon to the Panthéon is as follows: Once you've exited the Métro, take Rue de Condé or Rue de l'Odéon. The former runs right into the Luxemburg Gardens, and the latter runs into a big old theater building, with the Luxemburg Gardens right behind it; go around and put yourself in front of the Luxemburg Gardens. This edge of the Luxemburg Gardens is on Rue Paul Claudel, and you want to take that street, heading toward your left, hugging the edge of the park. In a few streets you'll arrive at a small circular place with a fountain in the middle, and Rue Soufflot will be off to your left. Glance down Rue Soufflot and you can't miss the impressive dome of the Panthéon at the end of the street. There you go!
Related junk from Cool Stuff in Paris:
My favorite cemetery, home to some very unusual tombs.St-Pierre-de-Montmartre
Church containing 7th century Roman columns.Les Catacombes de Paris
More skulls than you can shake a femur at.