I am in the northeast part of the state. There are good numbers of pheasants up here, though not quite as good as in past years. A lot of the land has come out of the Conservation Reserve Program and has been put back into crops. The quail population has not rebounded from the bad winters a couple years ago but there are a few around. The sw part of the state is supposed to have good number of birds also.
There are about 800,000 acres of publicly owned land (federal and state) open to hunting. In addition to this, there is another 115,000 acres in the Conservation Reserve Program- Management Access Program. This is private farmland enrolled in the CRP that is also enrolled in a state program that pays the farmer an extra few bucks an acre to allow public access for hunting. When you get away from the se part of the state (Omaha and Lincoln) or any of the larger towns, most farmers will allow access on their land as well. Information (as well as maps) about all of the land open to public hunting is available on the NE Game and Parks website
Pheasant hunting doesn't take much more walking than quail hunting, but a 1/2 mile through chest high switchgrass will take its toll. If you really want to wear out a pair of boots, try trekking across the Sandhills chasing prairie chickens and sharptails.
SD is most always good. If you are already planning a hunt there, I don't know if I would change plans, but depending on where you will be you might want to drop down across the Big Muddy and check out Cedar and Knox counties (south of Yankton).
I think I got you covered in the main text. If your guys can all wrangle an extra couple days they might enjoy it.