First find out what the rental has..
Such as crock pot, what kind of coffee maker or any other kitchen appliances that you might need.
Grill ? - Gas or Charcoal ?
Water toys, Beach chairs, umbrella or sun canopy, floats, sand toys, boogie boards, bikes, and beach towels.
Also inquire about the amount of towels, and kitchen towels and pot holders are in the unit. Nothing worse than try to get something out of the oven and realize there are no pot holders... Not a great way to start the day with burning the bacon...
Bring along with you:
Your own spice's, don't forget salt and pepper if you use them.
Drop in laundry and dish washer tabs are the greatest thing ever.
Lots and lots of good sun screen, nothing ruins a vacation faster than sunburn. The most important tip of all!
Flip Flops that can get wet, sandy, and are easily rinsed off.
At least 2 bathing suits... nothing worse than trying to wiggle into a damp bathing suit. Yucky...
For dinner out, nicer shorts, polo shirt... but really shorts and t-shirts, tank tops are all you really need.
Something for everyone to do inside in case of rain. Books, movies, puzzles, video games, the more relaxing the better.
Lots and lots of water, Gatorade... stay hydrated.
Have a great time..