Decker offers access to two state parks. Rosebud Battlefield State Park is the undeveloped site of the 1876 battle between Sioux Indians and General George Crook's infantry. One of U.S. history's largest Indian battles, it set the stage for an Indian victory eight days later at the Battle of Little Bighorn. The Tongue River Reservoir State Park offers scenic boating, camping and great fishing for bass, crappie, walleye and northern pike.