Comprising 200 acres, Alabaster Caverns State Park offers RV and tent camping in a setting of scenic beauty with many hiking trails. The main attraction, however, as the name suggests, is an exquisite gypsum cave, with its formations colorfully lit. Although it's claimed to be the largest alabaster cavern in the world that's open to the public, it can be toured in about 45 minutes. Other caverns on the site are available for wild caving and cave camping.
If you go:
Guided tours depart on the hour every day from 9 to 4.
Links:

217036 State Highway 50A
Freedom, Oklahoma 73842 USA
Alabastercaverns@Oklahomaparks.com
Phone Numbers:
Alabaster Caverns State Park
+1 (580) 621-3381
+1 (580) 621-3381