We recommend to stay on or close by Sandy Beach in Rincon, Puerto Rico.
No passport is necessary for traveling to Puerto Rico.
We've traveled to Rincon both with and without rental cars. If you're staying in the Sandy Beach area of Rincon, you'll likely be able to walk to many restaurants, bars, and beaches, and leverage the taxis/ubers in the area to travel elsewhere.
For the wedding: we're taking some liberties with "cocktail attire" - it'll likely be 80 degrees and outside. We trust that our friends will show up in their island best, finding ways to blend cocktail attire with island practicality (who doesn't love linen?). Is cocktail beach chic wear a dress code? If so: let's go with that. If wearing a dress is your speed, we recommend a dress length of your choosing. It's an island - so soft and bright solid colors and florals are appropriate and encouraged. Dinner will be seated, but there will be some standing and perhaps dancing on the beach - so shoes, while recommended, will likely come off at some point. For those who won't be dress shopping: we'd recommend a button up shirt and pants with (sorry, we know it'll be warm) a light jacket or blazer optional. For the welcome party: come in true resort wear as you would for any island restaurant (sun dresses, short or long sleeve button ups or collared shirts, shorts, etc.).
Aside from your wedding attire, Rincon is incredibly casual (t-shirts, flip flops, and swim suits are enough most days). If checking a bag, we'd recommend bringing your own sunscreen and bug spray. We always pack our snorkels and fins for some great snorkeling off of Step's Beach.