Hilton Head Island is very informal. There are few places you can't go to in shorts and a collard shirt. If you like to dress up for evening, one outfit should suffice. Ties and jackets on men usually means they are lawyers - not really needed for vacationers. Just bring lightweight shirts or tops, shorts and your bathing suit in the summer. You can even play golf in shorts (not short shorts).
In winter a sweater or sweatshirts will usually do. If you plan to walk the beach in winter, bring a windbreaker. Jeans will do for everything except golf.
Beach towels may not be provided where you are staying - be sure to ask. Beach chairs are for rent on the beach but consider buying them even if you leave them behind. Beach umbrellas are also rented on the beach and from private companies. Details here.
Bicycles are for rent for about $20 per week including helmet and chain lock. These are beach bikes with no gears - fine for a place as flat as Hilton Head. You can bring your bikes but you should know the salt air is very corrosive and if you ride on the beach your bike will age quickly. Also, bikes are not permitted inside hotels or condominiums, which exposes what might be an expensive bike to theft.
Since Hilton Head is a tourist destination, many items tourists need are available for rent. These include cribs, high chairs, playpens, wheelchairs, oxygen equipment, roller blades, etc. When you reserve accommodations, ask about availability of anything you might need and don't want to bring. Details here.
This page Updated 02/09/19