I just moved from Charleston after living there 6 years. THe drive to HH is 2 hours each way - so you may want to take the suggestion of spending a night there. It's a pretty drive though. It is a beautiful city. Compared to Savannah - Charleston has a more extensive historical district and a more things to do tourist-wise I think. Savannah has the waterfront area - but that's about it. The squares are pretty, but there's a historic house next to a parking garage next to a coffee shop. Don't get me wrong, I love Savannah because it's more of a "real" city than Charleston, but from a tourist perspective, it's hard to beat Charleston. It's a great walking city and probably the most european of amerian cities in that respect. King Street is fun to walk - lots of antique stores on one end, national chain stores have taken over the middle, and then some funkier local stores on upper King. The market ends at East Bay street which has some great restaurants and waterfront park is great and if you walk south a ways you'll hit the battery. There's a new aquarium that is beautiful, and a children's museum, if you have kids. And then about 10 miles out is Sullivan's Island which is, in my opinion, one of the most beautiful beaches on the east coast.
Now I'm getting home sick!
Have a great time whatever you do - we love HH and can't wait to go back.