A hidden gem of Northern Arkansas is Blanchard Springs Caverns. It is run by the National Forest Service. The cave is a good size and has beautiful formations. Three tours are offered. Only one was running when we were there in April. Be sure to check the hours before heading there. Our tour was lead by a great guide who had a good narrative.
Be sure to see the Springs before you leave the park.
You might want to stop for lunch at Cody's in Fifty Six (a town two miles west of the park entrance on Arkansas Route 14).