It depends on how old the kids are, how touristy vs outdoorsy you want to be and how much you want to spend on activities. For a family of 5, it seems like everything was about $50 as far as attractions go. Last time we went the kids were 9, 7 and 3.
Stayed in Spearfish on the way down and did Spearfish Canyon and the Boothe Fish Hatchery, Lead and Deadwoood on the first day. The fish hatchery is one of my favorite all time places from my childhood. I could feed those 20# trout all day long. Spearfish Canyon has some good hikes and some that work for all ages. Deadwood/Lead wasn't a hot spot for the kiddos in our family. Bases the rest of the trip out of Rapid City. We hit lots of places over the week. Free stops included storybook island and the dinosaur statues. We went to Bear County, Reptile Gardens, the Cosmos, Mount Rushmore, Crazy Horse Mt, Jewel Cave Nat'l Monument, Custer State Park, played some mini-golf around other stops, shopped in Keystone and went to the Thunderhead Mine. Kids really enjoyed panning for gold and garnets.
On the outdoorsy side of things, I could spend a week in Custer State park by itself. You need to drive the needles highway. I agree with renting a boat on one of the lakes or just doing some hiking and picnicing. I went on the Jewel Cave tour with my older two kids. Wife and youngest didn't go. They have a junior ranger program for kids at all of the national parks and monuments. This is a great thing to get your kids doing. They have to do some activities and gather some information. If they complete it at each stop, they get a badge. Kind of gives them focus and something to look forward too at the historical stops. I also recommend the Thomas Jefferson vanilla ice cream at Mt Rushmore. Made from his original recipe.
We didn't have time to hit the southern hills very well. Custer has lots of activities. Mammoth hot springs has the fossils and Evan's plunge. Loved that place as a kid as well. Then there is Wind Cave and Badlands Nat'l Park. Also forgot to mention heading over to the Devil's Tower. To see it all, you could go there for a week each summer three years in a row. I love the Black Hills and could go on and on.
My daughters and wife love the outdoors and my boys well it doesn't have a screen or controller or a football field not so much. I have never spent anytime there at all so I don't know much other then the Spearfish Canyon which I absolutely loved when I drove through. But man the few things I have priced out online....good thing I have been saving for awhile.
