If you're headed to Hilton Head Island, the weather there is not that different than Savannah, GA, where I live. You're looking at highs in the upper 70s to low 80s as the norm. If you're in the sun, it feels a lot warmer. However, this time of year can also have some swings in temperature, so be prepared for some cooler days, especially if it's cloudy. For the most part, I'd say it doesn't really start feeling like Autumn around here until early November.

I'd pack a good mix of options, leaning towards short sleeves and such, and check the weather report a day or two before you leave to confirm your choices.

Enjoy your trip!