I would suggest Folly Beach......you can park in the parking lot at the pier and enjoy the pier
or sit on a bench and enjoy the ocean without getting in the sand if that would be a problem.
If you wanted to walk around the Market area there is a lot (after banking hours) on the corner of
Market and Meeting. The cost is $8.00 to park and that might be easy for you to park and enjoy walking around downtown a bit.
Most of the stores like in Mt. P or James Island you can stop and take potty breaks.
Maybe you could take advantage of a carriage or bus tour and that would be easy on the leg.
Enjoy your trip!