I am from Rapid City and we travel back home every summer and sometimes more than once. Every time we go we take the kids up to Mt. Rushmore. There is a walking path up around the base of the Mtn. that isn't terribly rigorous but not stroller friendly. Also, be sure to go for the lighting ceremony at night.
Near Mt. Rushmore is the town of Keystone. I love to browse the shops. It's a tourist trap, but a fun one. Be sure to get some taffy from the taffy shop and let the kids watch the machine pull it. My mom loves the fudge that they sell in the shop there on the main drage (sorry--don't know names!) Also in Keystone, you can get on the Black hills Railroad
http://www.1880train.com/ It is a real 1880 steam train ride thru beautiful hills to Hill City. Actually, you can start in wither keystone or Hill City and go round trip or just go one way (if someone wants to drive over and meet you in the other town).
In Rapid City there is a museum, called the Journey Museum
http://www.journeymuseum.org/english/ I haven't actually been but friends have told me it is great.
Custer State Parkhttp://www.sdgfp.info/Parks/Regions/Custer/ is a great place to view the buffalo (they roam free) and I would recommend a visit to Sylvan Lake and possibly a hike to the top of Harney Peak. You can see five states from up there (highest pt in SD): WY, SD, ND, CO, NE I think.
It's been several years since I've been but Flintstones in Custer used to be a favorite. Crystal Caves, Wind Cave, Thunderhead Falls are also neat places to visit. Also, be sure to check out Pactola Reservoir and Sheridan Lake in the hills for some swimming.
So many more are coming to mind:
I believe Spearfish has the Passion Play every summmer, which is very moving.
http://www.theblackhillspassionplay.com/
Sturgis is the home of the motorcycle ralley. They will be preparing for the ralley when you visit and there will be several bikers in the area early. They are wonderful people and nothing to be afraid of if you do go at ralley time.
More favorites:
http://www.spearfishcanyon.com/intermap.htm Bridal Veil falls is free to visit and a great place to picnic. You can walk right up to the base of the falls and it usually isnn't terribly crowded as it's not right on the highway (but wasy to get to)
http://www.nps.gov/badl/ Please stop at the badlands--there are signs on I90 that tell you when. Doesn't look like much from the interstate but it is truly WOW!
In Rapid City:
Go to Canyon Lake Park and feed the ducks/ have a picnic/ride paddleboats
Take the kids to Storybook Island--it's free and very cute. Have a picnic there by the creek (there are usually plenty of ducks to feed there, too)
Walk to the top of Dinosaur hill to get a great view and see the dinosaurs (free)
Go to the Black Hills maze (we go every year and it is a ball!)
Go to the Ranch amusement park. Not cheap but fun. Also check out the Waterslides. Nothing fancy, but again, fun.
Got to one of the Black Hills Gold factories for a tour/to see it made
On Hwy 16 between Rapid City and Keystone (Mt. Rushmore) stop at Bear Countrywww.bearcountryusa.com/ or Reptile Gardenshttp://www.reptile-gardens.com/. My favorite is Cosmos (kids are free)
http://www.blackhillsattractions.com...ns/cosmos.html
I know that is a lot and I am sure I will come up with many more if given the time, but that is a good start! Let me know if you have any questions.
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