Based on the recommendations of the peoples house we were staying in, we all (6 people) went on the Lake Argyle Sunset Cruise (see here for details: http://www.lakeargylecruises.com/site/tours/#sunset
Highly recommended, our tour guide and owner (I beleive), Greg, was full bottle on information and facts and figures. His attitude and sense of humour was great.
The scenery was just fantastic and I even had difficulty trying to do it justice with the wide lenses I brought along specifically.
Again, the tour's highly recommended. Tell him the Argyle Mine people sent you, he'll charge extra!