Plums and apples. That makes me hungry... Yum.
Devil's Tower is really cool. Bring some good shoes so you can hike along the trails. They also have climbers and repellers that are fun to watch. (I hinted to my husband that I would like to train to do that sometime, the climbing part, and he didn't take it very well. Not the daring type and a bit on the protective side.

I just might figure out how to do that anyway. I've climbed indoor rock climbing walls (which don't compare), but those ones at Devil's Tower seem almost vertical, and very ridged. Repelled only a few times outside. That is the easy part I think.)
You should definitely try to see Mt. Rushmore. It is awesome. Lots of bridge-like walking trails (so bring those good shoes.) Plus the weather will be on the warm side (hat? and sunblock)
There is also a cool reptile gardens around Mt. Rushmore too. I loved it. Great if you have kids. The alligators and kamens and such are a bit stinky though, just to warn you. About blew me away. But so neat to see the trainer feeding them.
And there's a nice drive we did in South Dakota there in a canyon type place that is nice to see the fall leaves. Not good about remembering exactly where that was. Around Spearfish?
Have fun!!