I've been to Paris many times and never made it to the Pantheon, until May 2019. So glad I finally went. The main floor is amazing and the crypt is super cool. I mean, seeing the burial place of Voltaire, Mme Currie, Hugo, Balzac, Dumas! The audio guide was pretty good and free. Also, I thought the selection at the small gift shop was very good.
After the Pantheon, we walked to the Jardin des Plantes, stopping at the cute Place de la Contrescarpe for lunch. Many cute cafes around the Place. 5 minute walk from the Pantheon.