Complex sentence: The largest that is open to the public is Green Mountain State Forest, containing Green Mountain itself, a multi-use, over state forest owned by Washington State Department of Natural Resources allowing camping, off-road vehicles, horseback riding, and hiking.
Split into simpler sentences: The largest that is open to the public is Green Mountain State Forest, a multi-use, over state forest containing Green Mountain itself and owned by Washington State Department of Natural Resources. The state forest allows camping, off-road vehicles, horseback riding, and hiking.