I camped there in January 2013, site 331. Awesome campground. They have what is called Snow Mountain in the winter - they make snow and you can slide down on tubes. In the summer, there is a passenger train you can ride around the park. Bike riders galore - but lots of steep hills. You can visit am old graite quarry, and as stated, hike up Stone Mountain. It gets pretty steep near the top but our 4 year old (at the time) had no problem. Take the service raod part of the way down even though they say not to.
And - take the advice not to follow your GPS to the park. There is a low RR bridge I barley made it under. Can't advise you on the route to take - don't remember the roads, but it is a lot easier to get to the park - call for directions, or follow directions given on your reservation confirmation.
The roads in the park are OK, sites are gravel, some like mne have great views of the mountain. The park office staff is very helpful, and they can fill your propane tanks if needed. I would go there in the summer without a doubt.
There is a part of the park that seems to have residential TT's but it is kept up nice - and they pick up your garbage
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