Can't help much with the car rental but I will triple down on Airbnb... recently found a decent place 20 min. out of Hilton Head for $40 while my brother paid well over $300 for a resort room close to the beach, this even on a weekend. We also found a place for $65 at a stopover in Columbia that included breakfast for both my wife and I, was in a very nice home in a safe neighborhood. Both dog friendly, no less. You have to watch the extra fees but deals can be found!