Luggage and bags:
A bag to carry down to the beach is helpful, but if you forget grab one at Christi's or Chu's. Coolers are handy, but remember, NO GLASS on the beach.
Clothing/Shoes/Weather Gear: Flip flops - the pavement gets hot in the summer. The spring and fall weather is nice during the day, but if you plan to be near the ocean at night bring a light sweater or jacket. The breeze can get to ya!
Toiletries and Medical Supplies: Sunscreen is a must, even if it looks cloudy or hazy! That sun on Tybee gets EVERYWHERE!
Camping/Beach/Outdoor Gear: Lanterns and flashlights are helpful if you plan to be on the beach or back river at night. Fishing rods and a tackle box, if you want to fish (although tackle gear can be found at most gas stations).
Miscellaneous: Bring quarters for the parking meters. If you need anything you can't find at local shops or the IGA market, you can hit the WalMart and grocery store directly off the island (about a 15 minute drive).
Bathing suit, tank top and shorts are about all you are going to need.
Toiletries and Medical Supplies: SUN SCREEN!!!!!!!!!
My sister failed to put some on and she looked like a lobster!! Even though the sun isn't as strong as somewhere else it is still there.