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."
This is because the sentence contains items (cities) that are paired with additional information (states), which makes it clearer to separate them with semicolons:
"This summer, we are going on a road trip to Philadelphia, Pennsylvania; Baltimore, Maryland; and Rehoboth Beach, Delaware."