Don't assume your rental will have everything you want. Save space in your luggage for things like...
iPod and mini speakers
Laptop that plays DVDs In case the one on-site skips or isn't compatible with your disks.
Portable DVD player, video games, beach toys, stuffed animals If you're traveling with kids, you'll be happy you brought these items.
Pillows and bedding Helpful if you're particular or prone to allergies.
Air mattress Handy in case a bed is too firm, one bedroom has a musty smell, or the kids get scared and want to sleep on the floor in your room.
Groceries Buy basics online and have them delivered when you arrive.
Cooking spices Save space by putting a few tablespoons of your standbys in baggies; pack them in checked luggage to avoid TSA trouble.
Soap, shampoo, toiletries Some rentals might have travel-size amenities, but better safe than dirty!