Last fall my wife, sister-in-law, and I flew to Rapid City South Dakota for a driving vacation for two weeks. We toured the Dakotas, Montana, and Wyoming. When I say toured, that doesn't mean we saw everything in all four states but we saw a fair amount.
We visited Mt. Rushmore, Custer state park, Theodore Roosevelt NP, Badlands NP, Little Big Horn national battlefield, Deadwood SD, The Minuteman missile national monument, Old Ft. Meade SD, Cody Wy, Yellowstone NP, Billings MT, and other points of interest. I took a zillion pictures which I won't post all here, just some :)
There are several great firearms related places to visit, the Cody Museum of the West in Cody Wy being at the top of the list. If you really want to see everything in detail 8 hours wouldn't be excessive. Also in Cody is a small but very interesting museum of dug up firearms. No pictures allowed in it but it was worth stopping in to see it.

Ouside the Cody museum

Winchester revolvers




'Coffee Grinder' Sharps carbine

Yours truly holding a model 1816 Hall breech loading rifle



At the Cody museum you can book a guided tour that ends in the reference section where you may fondle some representative firearms.
At Mt. Rushmore

My sister-in-law (retired MSG USAF) and I along with several other vets in the audience retired the colors at the Mt. Rushmore amphitheater
