Tabor Hill is the easiest way to get above Hawassa and understand how the city sits against the lake. The view is better than the walk itself: roofs and roads in the foreground, Lake Hawassa beyond, and the Rift Valley landscape opening out around it.
Ask locally about the best access before setting off, and avoid going alone late in the day. It is a simple viewpoint rather than a major hike, but it adds useful shape to a visit that otherwise stays close to the shore.