Clarens is a cool little town. Plenty options with accommodation and restaurants, you got the Golden Gate National Park there. It's winter so it'll be freezing and not sure they'll have the option of River Rafting but plenty other activities you can do.
Overall for weekend, solid 5/7.
Obviously all depends where you're coming from - I wouldn't drive 10 hours to visit the place. But from JHB for a weekend, definitely.