Robberg is the essential Plett outing. The peninsula juts into the Indian Ocean with trails above cliffs, beaches, dunes, and a large Cape fur seal colony. In season, whales may pass through the bay; at almost any time, the views are worth the effort.
There are short walks if you only want a taste, but the full loop gives the place its scale. Bring water, sun protection, and proper shoes, because parts of the route are more exposed than they look from town.