Puerto Rico, St. Thomas, St. John and St. Croix are US Commonwealths which means the natives on these islands are considered US citizens. It also means that if you are a US citizen traveling to these areas, many of your US "perks" (insurance, cell service, etc. - be sure to check your carriers) are available as well.
These Caribbean islands allow US citizens higher customs limits so you may consider one of these stops for high ticket items like jewelry, etc.
If you are visiting St. Thomas, it is well worth a tour to neighboring St. John - the "nature island"! Sadly, St. Croix (where I used to live) is no longer that great of a place to visit.