You can do peddle boating at Pendleton Lake and fishing at the Blackwater River though remember that the section of the Blackwater River that runs through the park is a catch and release area. and that proper license and stamps are required. As far as places to stay, there is the Blackwater Lodge and the nearby Canaan Valley Resort.
You can do horse mountain riding or horseback and pony rides, if you have kids with you, at Circle R Ranch. Other spots visiting are Elakala Falls, the Pendleton Point Overlook (which offers nice views of Blackwater Canyon) and Lindy Point, which also offers great canyon views