I saw Mount Rushmore in the 1990s… family vacation with the kids. It was great !! We stayed overnight in a Cottage in the mountains… and got to see the Mountain both in the daytime, and again at night all lit up. IT WAS AWESOME !!
Lol, we were there in the summertime… and I remember it was really cold at night … in those mountains (near freezing). So I would summise that April will be very much WINTER !! Check the weather forecast / and look for local info to get a better idea of what to expect (TripAdvisor and their Forums could be a good source).
Since I was there, the Crazy Horse Statue has become a lot more than just “a dream”… so that will probably be worth a look see.
Deadwood… is also an incredible historic place to visit in The Black Hills… a mining town with a “colourful” past… (same Deadwood the HBO Tv show was based on). It very much still resembles an Old West Town with Saloons etc. Fun place. And historic being the place that Wild Bill Hicock was shot and killed (while playing cards and holding what has now become known as a “Deadman’s Hand”… Aces & 8s). Pay a visit to Boot Hill the town’s cemetery… and where Wild Bill, Calamity Jane, and other notable figures from the time are buried.
Coming from Dallas, you are gonna find this part of the country pretty desolate… with not a lot to see besides the incredible scenery. Keystone (pop aprox 300) is probably not the best choice for a “city gal”… Rapid City will have more to offer (pop aprox 68,000)
Here are some other ideas… if you have the time to see other nearby attractions…
Badlands National Park is aprox 70 Miles (1 Hour Driving Time) to the East along I-90 from Rapid City.
And enroute… you pass by Wall, SD home of the “world famous” Wall Drug Store (started as a “water stop” thru the flat prairie… and now a HUGE tourist destination, a tad tacky, but one of those Great American Roadway Landmarks worth a look see, if just to say you’ve been there !!)
The Minute Man Missle Historic Site is also located not far from The Badlands (at exit 131) for more info see the National Parks Service Website
Devils Tower – A unique geographical feature (and famous from the Movie… Encounters of the Third Kind) is located to the Northwest in Wyoming, aprox 107 Miles (1:40 Hours Driving time form Rapid City).
Enroute is the town of Sundance… home of the Sundance Jail where Harry Longabaugh served 18 months… and after his release in 1888, he took on the name “Sundance Kid”, there is a small museum to visit.
The majority of these attractions have websites… be sure and check them out, as you’ll need to know what to expect in April as to Opening Hours etc, as it is the Off Season.
Hope this helps,