Rapid City is the second largest city in SD. I consider the crime rate to be fairly high, by SD standards, so probably pretty tame for you coming from FL. There is TONS to do and see in the Black Hills and there's a reason Rapid City is known as the Gateway to the Hills: it's located centrally north/south. It's the perfect base of operations for exploring the Black Hills.
In no particular order, things I love in the Hills:
Highway 16 going south out of Rapid
Iron Mountain Road
Needle's Highway
Custer State Park (especially the Wildlife Loop)
Alpine Inn in Hill City (EXCELLENT legit German food)
Highway 385
Sylvan Lake
Pactola Reservoir
Rochford Road
Nemo Road
Sugar Shack south of Deadwood/Lead (dumpy place with OUTSTANDING burgers)
Spearfish Canyon
Crow Peak Brewery
Miner Brewery
Locals will be along shortly, I'm sure, with more.