Guess it depends on her age but in general: Provincetown for a day trip, stop at the National Seashore for an hour or two of beaching, happy hour at the Beachcomber in Wellfleet and top it off with the drive-in

That's one day. If she's in her twenties, there are a few good happy hours, Jimmy Plunkett in Dennisport, Pufferbellies in Hyannis... Chatham is a fun town to walk around and do some shopping, as is Mashpee Commons. Personally, I wouldn't waste any time in Hyannis unless maybe a walk down Ocean Street to the marina, maybe lunch on the water down there at Spankys or Tugboats. Again, if I knew her age I could direct you better but these are some good ideas