The "Hall of Marble" inside the Longhorn Cavern. By SARA VANDEN BERGE   I remember when our kids were young and we would search for something fun and affordable to do for Spring Break.   Our kids always enjoyed an outdoor adventure, so when The Husband and I paid a visit last weekend to Longhorn Cavern State Park , I thought, ‘Man, this would be a fun thing for local families.’”   Longhorn Cavern State Park is located near Burnet and touring its cave is like taking a grand ad