Definitely Crazy Horse over Mt. Rushmore. One is too commercialized and the other has far better secondary exhibits. Crazy Horse is supposed to be completed in June, they just don't know which year. They removed 2x as much rock as Rushmore and they have a long way to go yet. It might take them another 20 or 30 years to finish.
PS. if you find a pair of sunglasses in a hiking area just 2 min. north of Rushmore, they are mine and I would appreciate it if you could mail them back too me, OK?
PSS. If you've seen "North by Northwest" you might be disappointed to know there is no house on the top of Rushmore. Just thought I would add that so you didn't think the movie was real.

Definitely do the caves in the area. I think there are something like 7 or 8 really good ones in the area with 2 of them being counted among the longest in the world - Jewel and Wind caves. Jewel is probably the prettiest.

Don't forget the Mammoth exhibit in Hot Springs. I missed out on Deadwood but Devils Tower was a nice drive away from the central area. Note: it is bad form to be climbing the tower, it offends the natives who consider this sacred ground. I've got badlands in Saskatchewan so the ones in SD aren't much different. There are all kinds of historical displays, museums and such. Lewis & Clark museums for example.

The Reptile Gardens are really cool. K. Costner has his Tatanka exhibit by Deadwood.

There are even kitch stuff like the Flintstones Campground that appeal to family groups.

Balloon rides by Rushmore, biking, kayak/canoeing/rafting, rock climbing, etc. You could probably try panning for gold and get a few dollars of dust. Probably more stuff to do than you can fit into one week.