I lived there for a few years - well in Stellenbosch. This was a few years ago now, so there might be new trails etc. But my tips would be the following 3 which are near to CT. There is so much more, but depending on your time and transport options, these will get you going:
1. Jonkershoek - amazing place to ride, fantastic selection of trails. Talk to these guys for hire and tips:
https://ridein.co.za/
2. Stellenbosch itself - Coetzenberg was recently done up for the world cup XCO - its a fun course and has some really great features. You can also link up to the infamous G-spot trail (youtube it), and Paradayskloof/Eden which I think now extends out pretty far (might need a guide) chat these guys:
http://bmtsa.co.za/bmt-tours-trails/ about Stellenbosch rides and tours.
Also Dirtopia (just outside Stellies) has some good trails and hire I believe
https://www.dirtopia.co.za/
3. Contermanskloof/Tygerberg - closer to Cape Town -
https://www.tygerbergmtb.co.za/trails/contermanskloof/
Day trips to Wellington, Elgin, Greyton, Paarl etc can also reap rewards too! Tokai/Table Mountain trails I never got to, it burnt down while I was there, pretty sure its rebuilt though.
Safety is generally fine in all those places - once you are in them. I rode mostly alone in all those places. But leaving bike visible and unattended is asking for trouble.