How about going to public campgrounds that do not take reservations?
For instance, one of our favorite National Forest spots on a beautiful lake in the woods with great trout fishing has very few reservable sites - most sites are walkin sites. We just show up on Sunday afternoons or Monday mornings and can usually find great lakeside walkin sites at this particular place.
Also for RV'ing in the Western U.S. many beautiful and serene desert dry camping areas with no hookups are available. We like these for late Spring and early Fall camping in order to miss the summer crowds and hot weather.
It's been my observation that if you're set up for dry camping, in general many more opportunities are available for non-reservation camping all over the U.S. in the areas where we have been so far.
