You cold always stay with me in Goose Creek, or Toyman in Mt Pleasant (Mt P) and see how the (poor) locals really live. 
Hotels, well, the closer you get to downtown the more expensive it is. The reasonably priced bed n' breakfast does not exist.
The Boathouse out on the Isle of Palms is a great place. http://www.boathouserestaurants.com/ You can drink like a fish, eat good seafood and watch the dolphins swim in Breach Inlet. Best dive: The Wreck of the Richard and Charlene http://www.wreckrc.com/ Don't confuse this with another 'Shipwreck' in Mt P that (IMHO) sucks. The decor is, er, eclectic. 
Anything downtown is going to be pricey, which may or may not matter. Anson Restaraunt is pretty darn nice if you don't mind pricey. http://www.ansonrestaurant.com/
Just walk around downtown (avoid the carriage tours) and check out the houses. Go into the old churchyards and graveyards and look around; it's amazing what's in little out of the way places. Or even in not so out of the way places. Like the Hat Man:
Make sure you do this during the day, because 1) you'll see a lot more and 2) it's not so safe at night unless you are real close to the tourist traps. The Battery is cool up till around 11 PM or so, then the gangbanger wannabees and those who pursure the, er, alternative lifestyle pretty much take over.
Fort Sumter is worth the ferry trip, doesn't matter which side of the 'recent unpleasantness' you fall on.
Sullivan's Island was an Edgar Allen Poe hangout, he wrote 'The Gold Bug' while he lived there.
Folly Beach is called The Edge of America. I've never been to Key West but people who have tell me that Folly is a lot like it, maybe just not quite as 'anything goes'. I've been there many times, it's always been a decent day trip to just lay on the beach and do nothin'. If you get tired of laying in the sand, go up (north) to the lighthouse that's being stabilized. Cool to check out, if nothing else.
Not far from Folly is John's Island. It's so much like Africa or other exotic places that parts of movies set in those locales are shot there. The Angel Oak is there also. 
The picture doesn't do it justice.
If you drive up, try to take I-95 to where Highway 17 branches off (just north of Coosawhatchie) and goes toward the coast. It's called Charleston Highway and ACE Basin Parkway. It starts out 4 lane, then goes 2 lane through some of the most beautiful places you will ever see.
