Visiting Cumberland Island National Seashore requires some advance planning because you must reserve a site at Sea Camp, Stafford Beach, or one of the backcountry sites AND you must reserve a spot on the ferry from St. Marys that aligns with your camping reservation. Reservations can be made up to 6 months in advance and usually need to be made 5 - 6 months in advance for a site at Sea Camp.
I stayed on Cumberland Island in August and April. Months to avoid include July, August, and September because it is hot, humid, and later on in the summer - hurricane season! I did stay on Cumberland Island in August and the temperatures were record-setting hot almost everyday!!! The next time I visited I made sure to book the trip in April.
Sea Camp is a great way to ease into backpacking/camping/outdoor activities as it requires a short hike with gear to reach the campground but it is an established campground with (cold) showers and toilets and there are wagons available to help you haul your gear. In terms of packing, bring what you would bring for car camping but remember you are there to enjoy the beach and not to spend all day hauling gear from the dock to camp. After two trips, I've found that a backpack, cooler, camping supplies box, and chair are really all I need. Some campers bring showering systems to obtain hot water showers - I love the cold water showers especially after a hot day at the beach.
Once at Sea Camp, be sure to properly store your food inside the provided food cage AND make sure your food is properly wrapped and stored - ants can be problematic. If you have a cooler, you can buy more ice from the Ferry each morning at the Sea Camp Dock and you should plan to buy ice each day if you are staying for the hottest months.
I recommend a minimum of two nights but if you can stay for a week I highly recommend it. Staying multiple nights guarantees isolation and peace and quiet at night as the day visitors leave AND Cumberland Island is amazing for star gazing and staying multiple nights gives you a better chance of seeing shooting stars.
In terms of what to do - be sure to take a sunrise walk at least one morning - I took a sunrise walk and found SO MANY treasures washed ashore over a small area - I left them for others to enjoy but not until I lined them all up for a photo (see below). Also be sure to hike to and explore the Dungeness Ruins and make it a roundtrip hike by returning via the beach.
Finally, if you are only there a few days I would stay on the southern end of the island but if you are there for a more than a few days I suggest renting a bike (from the Ferry operators) and biking up to some of the other historical areas on the island.
In preparation for your trip, I highly recommend reading Untamed as it tells the history of Cumberland Island and the stories of some of its most famous residents.
Finally, be sure to hike some around sunset to see armadillos and wild horses.
I camped in March 2016 prior to Hurricane Irma.
The campsites are approximately 5 minute walk to beach, camp store, dining, and bus stop.
Beautiful scenery - absolutely paradise.
The campsites are sandy with soft ground and have a picnic table.
You can walk to several attractions from the campground.
NOTE: Currently the campground is closed due to Hurricane Irma. The Hurricane destroyed many historic buildings in and around the campground.