If you are in to dispersed camping (a.k.a. boondocking) there is a multitude of good places to camp off of Forest Service roads. Stop at a Forest Service information station and get both Motor Vehicle Use Maps (there are two, east and west). It will tell you where dispersed camping is allowed. Seems the only time the backcountry is crowded is during hunting season. Some roads are good, some are bad, all are dusty. I have done a lot of camping between US85 and US385 during the height of tourist season and had the place practically to myself; some mountain bikers and horse back riders, thats about it.
Spearfish has an excellent city park where drinking water is available and has wading in tame rapids, if you have kids or are there are looking to cool off when it is hot. Be careful in Spearfish if you leave a pet in your vehicle as they have very aggressive animal welfare patrols.
Don't go during Sturgis Bike Week (unless you enjoy crowds) as they dominate the tourist attractions for about 2 weeks before and after: RV campgrounds will be full, you will wait 1+ hours to pay $0.50 higher per gallon for gas, all parking spaces will be filled with bikes, left turns from side roads onto a hiway is next to impossible... I only saw two bikes on FS roads the weeks I was there last year during Bike Week.
jim