Even if they don't want the tour, they may want to visit Ft Pulaski and just walk around. It's a beautiful spot. Dolphin tours? Yes you can do that on HHI...you can even take one here where you can see HHI (with a good pair of binoculars, I THINK you can see the Beachhouse from Tybee).
Anyway, if they are in to things like Golf, we've got that. River st and City Market will have live music many times, and, the local spots almost always have someone (but the 19 y.o. may have trouble getting in some of them). There is LOTS of great food. I HIGHLY recommend Mrs Wilkes, but, be aware that you have to wait in line to get in, and, it's "Family Style", so, you sit with the folks you waited in line with. (Kind of like the German restaurant in EPCOT).
There is a waterpark in Pooler which may be easier to get to than Savannah from HHI. There is an IMAX theater there too.
If nothing else, tell them to go to Leopold's Ice Cream and get a few scoops. VERY good ice cream.
The main issue is that the main point of Savannah is Historical Tourism. They don't call it Slow-vannah for nothing.
Write me back if you think of anything else!