Yeh, the Badlands are great. Lake Sakajawea is pretty neat too, unfortuantly, I'm unable to spell it correctly. Bismarck-Mandan has some semi-interesting Ameri-Indian stuff - "The Depot" in Mandan - Native American Art and whatever it is the earthlodge thingys @ Fort Lincoln are called.
The Missouri is also quite beautifull around Bismarck and has some great mountain biking. If you like golf (I don't so I'm not tottally sure), Hawk Tree, north of Bismarck is apparently quite good / unique nationally, Bully Puplpet (another golf course, this one in Medora in the Badlands) I'd imagine is also pretty neat.
I'd fly into Bismarck, and stay there for a day & then go to the Badlands. Advice on Bismarck-Mandan: I'd eat supper at the Walrus - Have a Moose Drool on Tap there, the Bistro, Pirouge Grill, or Meriwethers (can't make up my mind

), East 40 is also really good. I'd then have a drink @ the Broken Oar in Mandan next to the Missouri. For lunch, have an Indian taco (if you come to ND, you need to have an indian taco) & a Margaritta @ Fiesta Villa for lunch.
Whatever you do, don't miss the Badlands.
I'd ignore the eastern portion of the state; Its boring as hell.