I'll second the Green River/Interurban. The Interurban gets a little straight and dull after southcenter but you can really get going on those flats. Although one ride I did down there the headwind on the return was brutal. Alki trail isn't bad on the Harbor Ave side, but once you get around the bend toward the beach proper you kind of have to ride in the road since the pedestrian traffic gets a little oblivious to the bike trail in the evenings. You could combine it with a Water Taxi trip and catch the Alaskan Way/Waterfront trail, not the most efficient but a nice tour.