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, semicolons are needed to clearly separate the items in the list because each item includes a city and its corresponding state. Thus, the corrected version would be:
"This summer, we are going on a road trip to Philadelphia, Pennsylvania; Baltimore, Maryland; and Rehoboth Beach, Delaware."