"Der Senior fährt in viele Städte."
Because it's in the accusative case. If it were 'Der Senior fährt in vielen Städte' then you would be correct.
In is one of these prepositions that can be followed by accusative or dative, depending on if there is movement involved or not (movement meaning from A to B; movement within a location doesn't count). So the adjective ending indicates the case is accusative, and therefore the senior must be traveling to the cities, whereas your translation suggests he's driving within the cities.
Hope that's right and hope that helps!