The sentence that requires semicolons instead of commas is:
This summer, we are going on a road trip to Philadelphia, Pennsylvania, Baltimore, Maryland, and Rehoboth Beach, Delaware.
In this sentence, the semicolons would help to separate the city names from their respective states to avoid confusion. It should be rewritten as:
This summer, we are going on a road trip to Philadelphia, Pennsylvania; Baltimore, Maryland; and Rehoboth Beach, Delaware.