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 locations have additional information (the states) that creates potential confusion. Using semicolons would clarify the separation between the cities and their respective states. The corrected version would look like:
"This summer, we are going on a road trip to Philadelphia, Pennsylvania; Baltimore, Maryland; and Rehoboth Beach, Delaware."