Rapid City in October

May through October gives you the best weather, but timing matters more than you'd think. June and July bring crowds and $200/night hotel rates during peak season. September hits the sweet spot – warm days, cool nights, and the aspens turning gold in the Black Hills. October can be gorgeous with fall colors, but you risk early snow that closes mountain roads. May and late October offer the best deals, with hotels dropping to $60-80/night, though some attractions have limited hours. Avoid Sturgis Rally week in early August unless you're into motorcycles – the entire region gets overwhelmed with 500,000 bikers, and everything costs triple. Winter brings cross-country skiing and snowmobiling, but many tourist attractions close November through March.