I've never camped in Stone Mountain but I did have an annual pass and will probably get one again when I want to do multiple events there. First off it is not a theme park although as stated previously they would love to make it one. I think this is a good thing because it forces the park management to be very creative in the offerings they have in the little village and also forces them to think of events to host instead. It's mostly a presentation of crafts and various shops. At Christmas there are cute live shows and along with the regular acts on the weekends.
I dont know if there is a special ticket for the Christmas and Halloween shows because I've had the annual pass any times I've attended. However I don't believe there is a special price for Christmas except possibly lower if you want evening only. One tip to keep in mind is that you can enter the village without a ticket. You will be able to see the demonstrations, live shows & street performers that aren't in the big tent plus the snow queen grand finale for free. The DUK rides are a separate charge.
What I would do is run the numbers. The family membership may be cheaper if you plan on doing everything a second day (tram, train, 4-D, plantation, the treehouse and big barn...and maybe miniature golf). The tennis facility is no longer operational...a separate stadium was built outside of the park and is now run as a teaching center. You can get lessons and play for a fee, however they might be closed at the holidays. You might ask if there is a tennis court over at the Marriott you can use. There is golf and miniature golf (there may be miniature golf over at the campground as well but I don't recall it).
From a state park point of view there are a variety of trails you might want to do. For the best view there is a walk up trail to the top. The trick is to saunter up...you'll be much happier. Don't be fooled by the people who walk up it for exercise. Or you can ride the tram.
If you go out the West gate you will go into the Village of Stone Mountain (as opposed to the Crossroads in the park). There are a couple of places there that bicycles and there is one that will rent scooters to licensed drivers. I thought the bike places were pricey but if you dont plan on keeping it long it might be fine. The
scooter place I haven't had a chance to price. If you like German food there is a German bakery and tavern plus a handful of other restaurants you might enjoy. Some have live music at times.
As far as food shopping and what not the easiest thing to do is continue on 78 eastbound until it ceases to be a parkway. The last exit is East Park Place. If you turn right there is a Super Target...and the Tennis Facility. If you turn left and at the second light turn left you'll find a
Walmart Supercenter.
Should be obvious I live close enough to spend time there but I can't help with regard to the campground itself.