What is the fine-grained topic of the following text: Polk is known for its water falls, biking, hiking, snowmobiling, canoeing, rock formations, boating, river fishing, hunting, public land, hills, valleys, bluffs, big views, sailing, cross country skiing which also has Sand Creek State Fishery Area, Straight Lake State Park, Sterling Barrens State Natural Area, Big Butternut Lake Park, Balsam Branch State Wildlife Area, Cattail State Trail, Bone Lake, Wapogasset Lake plus a couple communities including; Frederic, Osceola, Amery, Cushing and St. Croix Falls. Only return the topic.
Outdoor recreational activities and natural attractions in Polk.