Sunset on a beach is about as good as it gets, and a beach is a perfect setting for a tropical party, if your chosen beach allows fires, or has fire pits, all the better. If your budget allows, a portable shade pavilion, the kind that you put up like a tent with no sides is a good place to center the proceedings. You can find these at your party supplies store, in various sizes. The shade will help keep food, people, and beverages cool and will provide a place to get out of the sun for a little relaxation. Put down a large blanket with some lounge type beach chairs, and make sure that yours is closest to the cooler.
If you are lucky enough to have a fire pit, remember that the wind will change directions at sunset so pitch the pavilion accordingly. If your tropical party is to last through the day and into the evening you will need enough food for everyone for two meals and constant snacking. If there is a BBQ pit or if you are bringing a camp stove, then one meal can be cooked on site and the rest of the food can be pre-made and brought in an ice box. You can pre-mix margaritas and mai-tai's, remember it is illegal to throw a tropical party without both of these beverages, but making them on site without electricity is problematical, bring plenty of crushed ice, a warm mai-tai is just not right!
Don't forget the essential party accessories, paper plates and party cups, these can all be bought with a tropical theme and are a necessary addition to any outdoor party. One other item to remember for any outdoor party not just a tropical party is a rubbish bag, please clean up when you are finished, I might be there next.