Luggage and bags:
A backpack or a lumbar pack with water-carrying capacity if a good idea in summer.
Clothing/Shoes/Weather Gear: The weather in summer is generally very warm during the day and pleasantly cool at night. The Black Hills do experience sudden thunderstorms, so some kind of rain gear is a good idea, even if it is the $1.99 rain poncho from a convenience store.
Photo Equipment: I would bring some black and white film...there are a lot of Ansel Adams moments (of course on a smaller scale) to be found in the Black Hills.
Camping/Beach/Outdoor Gear: Bring water bottles and a big water jug. Most of the State and National Forest Campgrounds have a water spigot, but no shower facilities. One of those 'Solar Showers' would be a great thing to bring.
Miscellaneous: Bring your mountain bike or hiking boots. There is so much public access to land out here that you can't see it in one trip, but it sure is fun to try.
Camping/Beach/Outdoor Gear: Please see my camping list on my South Dakota page. All I will add is that it was VERY CHILLY AT NIGHT. We wore several layers of clothes and zipped together the sleeping bags.