Carriage tours are always fun unless you're a local who is driving to an appointment and find yourself stuck behind the damnable things.
Tours are informative and interesting. You'll learn so much about Charleston history and general history of the South. Tours last approximately an hour or so, and you'll be crammed in there with about 10 other tourists as well. Get a large group together so that you can hog the whole thing.
The horses wear diaper-type things so that way they don't poop in the street.
If you happen to be downtown (anywhere the carriages go) and you see an orange ball with a flag on it laying in the street, DO NOT PICK IT UP. It is a marker that the carriage drivers toss out when the horses pee on the street so that sanitation knows where to hose down. As nifty as they look, they are NOT souvenirs.