We got lucky and last second acquired a tour for the Pantheon, and it was a great surprise with how good it was! They do offer reservations for free entry but you have to book a time slot through Rome's official page and it does not include a tour. We found the tour on GetYourGuide, and like I said it was fantastic. We never would have learned facts about how the ceiling is a giant sundial on steroids where based on the light of the sun, they could not only tell time, but day, week, and month!! Crazy!!