Solo, City Walls Walk works.
Walking the full circuit is genuinely awe-inducing: sea on one side, the toy-town of stone and orange roofs on the other. The honest catch is that it's expensive — €40 in summer now — with almost no shade, and in July–August you shuffle in a slow-moving conga line behind cruise crowds in brutal heat. Go at opening (8am) or the last 90 minutes before close to dodge both the worst crowds and the worst sun. Off-season it drops to €15.