Witchita Mtns would be awesome.
I think there are some places in SE Ok on the Wyerhouser land that the ODW leases. I don't know for sure how many acres it is but I'm thinking its in the hundreds of thousands. See if there is anything around the Talehena drive area, that ends up in Mena Arkansas. Hanobia?
I didn't know you could legally camp anywhere that wasn't an improved campground any more!? Awesome! Let me know when you find out which places allow it. Otherwise, I can give plenty of places to make solid day trips!