We rented a house on Harbor Island in 2008. I had a really hard time finding any information about it before we went!
It was a fun week. We were looking for a very quiet place and that is certainly what HI is. We had one of very few homes there with a private pool so we spent most of our time in the pool, which isn't very helpful to you. However, IIRC, there is a very nice clubhouse with a large community pool there. The kids loved driving the golf cart around. Not very many places to eat nearby - you have to drive a ways. We ate at Johnson Creek Tavern (I think it was called?), which was fine. Never ate at the Shrimp Shack, but I wish we had. The most fun meal we made at the house used shrimp we bought at the Gay Fish Co., which which was used for filming scenes from Forrest Gump. It was so good and not expensive! Watching the shrimp boats all week was major entertainment. We spent a day at Hunting Island State Park, to see real waves and go up the lighthouse. (The waves on Harbor Island are very wimpy because it's an inlet.) The lighthouse was not expensive to go up, I don't think. There's a nature center past Hunting Island almost at the bridge to Fripp Island that has programs for kids. We went there to check out the exhibits, which are free. There's a boardwalk around there that is free, too. Oh, shelling on both Harbor and Hunting Islands is pretty fun.
We took several day trips into Beaufort for restaurants and some historical attractions. It's a lovely little place that is overlooked. The Sheldon Church Ruins are worth looking at too and free, if you are interested in that sort of thing. We stopped by them on our way home. I can't answer any questions about Hilton Head, as I've never been there. I've been to Charleston and Savannah and absolutely love both cities. We actually spent time in Savannah as part of our Harbor Island vacation. The city market in Charleston is fun to see and has cheap souvenirs, and of course it is free to stroll along the Battery.
Such a fun vacation! Now I want to go back!