Crazy Horse is very expensive, you can see it just fine from outside in a small parking lot.

As mentioned, Devil's Tower is a highlight, but will take a half a day from the Black Hills area to see properly. Mark sure to do the Joiner trail.

Custer State Park wildlife loop.

If you arrive a half hour before official opening, you can park at Mt. Rushmore for free in the garage. The only fee there is the parking fee and it is not covered by any National Park Pass.