White water rafting - orange river is good
Horse riding - almost anywhere in SA
Mountain Climbing
Surfing or Wind surfing
Absailing
Sailing
Bungee Jumping
Paragliding
Soooooo much! Try www.nomadtours.co.za and www.kumuka.com for a start. I was an overland tour guide and these kind of treks are great value for money and great to meet other hardcore travellers.