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 should be used to separate the items in the list because each item includes a comma. The revised sentence would look like this:
This summer, we are going on a road trip to Philadelphia, Pennsylvania; Baltimore, Maryland; and Rehoboth Beach, Delaware.