There are a few short hiking trails in this area - and since you're so close to the city, it's quite difficult to get very lost. Take at least a litre of water along per person, a warm top, and a pair of closed shoes. Also, hike in groups of three or four... our nature borders on a city, after all.
Save, and keep handy, the Mountain Club of South Africa's number (+27 21 937 0300) as well as the number for crime-related incidents (+27 86 110 6417) on your mobile phone. Those numbers may come in super handy some day. There are a few other numbers here, but they seem to be two individual's personal mobile numbers, so I'll rather just link to the MSCA page.