There is quite a bit here from light up to 5+ trails, although I haven't had time to find too much myself because I spend most of my time east out on the pierre shale. But in the hills any trail marked is open. Lots of area will be opening early in May that is currently closed. I think on the natl grasslands you can drive anywhere.
East of the hills three is a place called railroad buttes (south of Farmingdale) that is the same as the badlands but on natl grassland, but it can get really slick if it rains.
Sorry I'm not more help, When do I get a chance to go out I usually just take whatever looks good.
Here is a link to a local 4x4 page that should be able to help you more. Lots of jeep and toyota people around here.
http://www.blackhillsbadlands.com/external.asp?url=www.bh4heelers.com/mission.htm
and forest service maps
http://www.blackhillsbadlands.com/external.asp?url=www.fs.fed.us/r2/blackhills/maps/
Other than wheeling there is good mtn biking, hiking, climbing, fishing and you can drink in state parks.
Most if not all of the attractions are tourist traps, the badlands and Spearfish canyon are worth seeing, the needles hiway is a nice scenic drive, as far as Rapid City goes, not the coolest place but its okay.