Cape Dirt in Klapmuts. From the R44 direction Wellington, turn right onto the Agter-Paarl road that leads to Fairview. Cape Dirt is on the first farm on the left.
I did an off-road riding course there, and it was lovely to see whole families come to spend the day. Dad and kids zooming up and down and moms either playing along or packing out a picnic and relaxing in the shade.
They have various kinds of terrains, small bumps and stuff, and enduro track heading into the mountain, and a restaurant that serves tasty hamburgers etc. Various courses are offered there - MX, Enduro, Adventure etc. They have accommodation if you want to stay over, too.
Speaking under correction, but annual fees are R580 and you pay R100 per day to go play. R100 only for the whole day when you go for the first time.