We visited Hilton Head for Christmas, so it was winter and therefore too cold to go swimming or various other water activities. Even though your choices were limited, there was plenty to do. We rented bikes, and since our hotel was near the beach, we rode there most of the days we had the bikes. It was lots of fun chasing the seagulls around, and skidding through the sand at high speeds. We saw lots of other people having fun also. People went fishing, flying kites in the high winds, collected shells, and there were a lot of really neat sandcastles (view pics). The beaches were also very clean. We saw no trash lying around, and the water was fairly clean too.
The sand is hard-packed near the water, and we rode about three miles along before turning around. It looks the same as far as you can see in both directions. The guidebooks say that there are 12 miles of beaches. I'm sure that in the summer, the beaches are more crowded.