Lawton Stables does trail rides still, I think. my friend no longer works there so I don't know about it.
If you bring your own horses, you can go to Hunting Island State Park nearby.
Otherwise, check out Lawton Stables for rentals.
There are a lot of good barns around HHI and Bluffton. Most of them are boarding stables. Former jockey Maples runs one or two of them. Most don't allow rentals (most will do lessons), but Lawton does trail rides.
HHI is the most beuatiful island around. We always drove over there daily, rather than use our old Tybee Island each summer.
Lawton does trail rides and will do private rides but they are pricey (last I knew $100 per person). You're not allow to trot/canter "technically" but if the guide knows you can ride, they'll allow you to.
Other than that there's no real barns that will allow hacks/trail rides to the public, although there are some great lesson barns.
Please PM me with any questions. I lived there for 21 years
ETA- I used to work at Lawton several years ago and still know some employees and the owner there so I can get you some more information if you're interested.
I may know of a few options for a more "advanced rider friendly" trail situation. Shoot me an e-mail. I think the key is to get in with a barn via lessons rather than through advertised trail rides. My take on the HHI area is that it caters to the one time out-of-towner types that are only looking for the usual walk nose to tail ride. There is extraordinary countryside in the area and lots of awesome wildlife that is much better viewed by horseback.