For this trip, swimsuits and a sun hat are a must! (I don't typically bring a hat like this with me unless I'm going to be beaching. Also, I'm bringing two swim suits, because I've been to humid climates where suits don't dry overnight, and there's very little more annoying than putting on a wet bathing suit!)
Now, that's sort of a loaded question. I'm a firm believer that what you pack is pretty personal and varied, so this list is what I would bring with me. In addition to that, what you pack is also pretty dependent on where you're going, and when. Perfect case in point: I'm packing for a spring trip to the Greek Islands. If I were packing for a fall trip to London, my bag would be drastically different! My best advice is to take your itinerary (even if it's rough!) and try to figure out what you'd be comfortable in for each day/adventure.