Charleston is pretty walkable around King... from Hyatt to all the shopping/restaurants, the City Market, Rainbow Row, and the Battery (probably 30 min walk one way total). We've parked and walked it a few times. We've never had an issue finding free on-street parking. But if you plan on leaving the historic area, a car is best. Magnolia Plantation is 30 min drive. We've been to all the beaches you mention multiple times. Our favorite is Isle of Palms. Parking is VERY close to beach access. There's several restaurants, ice cream shops VERY close to the beach, so we'll always walk out and grab a cone or some food while spending the day. Folly is good too. Sullivan's has nothing close to the beach (restaurants or shops) and is much more a residential area, although this also makes it the least crowded. It's our least favorite out of the three. Either Folly or Isle of Palms is at least 30-40 min drive from downtown Charleston, and could take an hour during Memorial Day weekend, so I can't imagine an Uber would be very cost effective. In summary, we've never felt like a car was a burden in Charleston. We've been to many major cities, and lived in NYC for over 10 years. I would say NYC, definitely burden to have a car. Miami, San Francisco, even LA at times, definitely a burden. Charleston is VERY small in comparison and a car has always been beneficial. It has also been extremely hot here recently, and it's always hotter at the coast. Even though some places are only 20 min walk from the hotel, you may not want to walk in the heat. Just my opinion

We drive to Charleston 3-4 times a year. The only place we've found it difficult to park is around the Starbucks/Apple Store and within those few blocks... always crowded.