Yep, forget the bike rental.
By car there are some interesting spots, depending on what is important to you. To start off: North-east of JHB is the Pilanesberg Game Reserve.
Eastwards is the Kruger National Park, and on the way there don't miss the scenic roads where the escarpment crumbles off from 1,700m to 300m. Scenic points are Blyde River Canyon, God's Window, Pilgrims Rest etc.
South of JHB is Lesotho, but this means gravel roads. With a 2x4 SUV no probs. An exceptionally scenis road is from Oxbow down to Mokhotlong, then westwards via Thaba-Tseka to Maseru and north again. Just don't try the Sani Pass except if you've rented a real 4x4.
Only when we pause to wonder
do we go beyond the limits of our little lives.