Crazy Horse Monument is a must see!!! we have been many times thru the area. If you like off the wall kind of things I recommend "The Mystery Spot" and Bear Country USA. All relatively close to Mt Rushmore
This is a personal opinion and is not that of any other member of this site.
I have been to SD a number of times, as Custer State Park is gorgeous! I have done the 1880's train, which is a must do while there, as well, as Crystal Caverns, Wind Cave NM, and Jewel Cave NM. All are awesome.
As I say that, don't waste your money paying to get in to see Crazy Horse. I have twice, 7 YEARS apart, and after we got home, my wife and I were talking about the trip and we both mentioned how CH had looked almost identical to the first time we were there. So I dug out the photos I had taken the first time and compared them to the second time. There was NO visible difference in the 2 sets of photos. The only difference was the cost to get in. $8.00 the first time, $20 the second. The only major change to the whole area, was a brand new 2 million dollar tourist trap, er, I mean Visitor Center. To get to the outdoor viewing area, you MUST walk the entire VC, past all the "Buy Me Now" trinkets and other stuff. IMO, this whole thing has become nothing more than a money making scheme for the family of the original sculptor.
And, if you think I am being obnoxious, has anyone seen OZZY and son's new TV show? Well, one of the places they visited, last year, I believe, was CHM. They were able to get out to the actual monument and stand on the top of the arm. From everything shown in that episode, they have made 0 progress. Based on what I personally have seen, it will take over 100 more years for them to complete the sculpture to the original specs. I do understand that this is privately funded, by you, me, and everyone else who pays to get in, but, they claimed over 1,000,000 visitors in 2012, the last time we were there. At the rate of $20 bucks a pop (car) and an undisclosed amount per bus, conservatively, they made well over $10,000,000. Why is there not more progress?
Don't take this the wrong way. CHM is an awe inspiring undertaking. The sculpture is HUGE! And, much larger than Rushmore. But you can see it just as well from the road, get the same pics from the road, and not have to pay their demands to do so.
Again, these are my opinions, not those of anyone else, as far as I know.