There are several threads (and websites) available that discuss this subject in detail. Just do a search if you're in a hurry.
Additionally, some of the easily forgotten "essentials," in my opinion, are:
* Bug spray
* Sun screen
* Extra bathing suits
* Over-the-counter and prescription medicine (cold & allergy meds, asprin, etc.)
* After-sun lotion or sunburn spray, such as Solarcane (sp?)