I lived there for a couple years amd I can say that there is something for everyone...Spearfish Canyon, Iron Mountain Road and Needles Highway will give beautiful drives with sights along the way. The Wildlife Loop is best avoided say many locals, as the drive goes on forever and it is rare when you do see something.
I liked the natural sights and historic places. I enjoyed Roughlock Falls, Cathedral Spires, caves, & Wild Bill's graveside in Deadwood, the very historic look of many of the buildings in the small towns.
The less travelled caves (like Windcave in Custer State Park) are less expensive and are wonderful.
Mammoth Dig in Hot Springs is very pricey and so is Evans Plunge. If you want to swim in the naturally warm water, take a drive south of the town to a natural swimming hole. I heard it was changed some and I have not been back to see what it is like. It used to be nice...and free. At the bridge south of the Plunge, you will see large goldfish hiding in the weeds. These are not native to the area, but interesting.
Here is a link to National Geographic:
http://travel.nationalgeographic.com/travel/road-trips/black-hills-south-dakota-road-trip/
Here is a link to lakes and reservoirs:
http://www.howtoenjoytheblackhills.com/recreation/lakes-and-reservoirs/
Sylvan Lake is beautiful as is the reservoir at Hot Springs (NW of the town). I would say to check out a winding trail to the west of Evans Plunge. There is a beautiful park out that way. While you are exploring, check out their cemetery. It is absolutely stunning and the epitaphs are interesting.
There is a hike you can take to the highest point in the Black Hills, Harney Peak. I never went on it but I have heard it is rewarding.
Here is a link to the trails:
http://www.fs.usda.gov/detail/blackhills/recreation/hiking/?cid=stelprdb5190237
There are a lot of tourist traps, many gimmicky, but if you choose carefully, you might find some are worth going to. It all depends on your tastes.
There are rattlesnakes and black widow spiders in some parts, so be aware and alert.
Interact with rhe locals, ask questions about hidden treasures (they are often the best) & have fun!