American citizens need only a valid passport to enter South Africa for visits of up to 90 days; this includes infants. Check the expiration date. If your passport will expire within six months of your return date, you need to renew it in advance, as South Africa won't let you enter with a soon-to-expire passport. You will be denied entry to the country if you do not have two blank, facing pages in your passport.
Before your trip, make two copies of your passport's data page (one for someone at home and another for you to carry separately). Or scan the page and email it to someone at home and/or yourself.
U.S. Passport Information
U.S. Department of State. 877/487–2778; travel.state.gov.