Tengo entendido que cuando alguien está en el hospital y está ingresado se dice: "She is in hospital". Sin embargo, cuando alguien va a visitarle se diría: Tell him we will visit him **at the hospital **tomorrow afternoom.

Tengo entendido que lo mismo sucede cuando alguien está "in prison" es decir, que está ingresado en prisión. Y cuando alguien va a visitarle **"at the prison". **

¿Sabe alguien algún ejemplo más de este tipo?

In US Engish we say in the hopistal when referring to someone being a patient. British English or European usage uses the term "in hospital" referring to being a patient at some facility.

You could avoid having to choose between "in hospital" and "in the hospital" by using the passive construction "hospitalized" or "imprisoned". With "I'll visit him (in prison | in (the) hospital | at the prison | at the hospital), I think that there's a subtle difference in meaning. The use of "at" means that you are simply mentioning the place where the visit will occur. Using "in", on the other hand shifts the emphasis to the circumstances of the person that/whom you are visiting (although it also implies, of course, where the visit will take place).

You get in a car but on a plane, though you sit inside both. Probably rooted in the tradition of boarding or being onboard a ship.