Are non-Muslims allowed to visit the Prophet's Mosque in Medina, assuming you are allowed into Saudi Arabia in the first place?
Wikivoyage mentions that non-Muslims aren't allowed into central Medina:
WARNING: Non-Muslims are strictly prohibited from entering Central Medina. The minimum penalty is deportation from the country. Documentation will be checked upon entry and anyone not showing proof of being Muslim will be denied entry. However, many parts of the city, notably the outskirts and the Medina Airport, are open to all.
but it only explicitly talks about which hotels are within central Medina, as opposed to the mosque.