If you're looking for a good place to take sunset photos or watch the sun go down - this is it. Its free to enter and you can do a short walk around the top of the hill in about 20 minutes.
Bring a picnic blanket, some snacks and take it easy atop the hill. There are also plenty of rocks to sit on as well as benches. It's also a really nice to place to bring a bunch of friends as there is lots of space.
Best to get there around 60- 80 minutes before the sun goes down as it can get quite busy and the parking fills up. There is more parking just down the road though and you can park on the street. Don't leave any valuables in the car (or at least put them in the boot/trunk).
For the history fans, Northcliff Hill is an interesting part of Johannesburg's rich history. It has hosted inhabitants since 500 000 to 250 000 years ago. Stone tools and settlements from hunter gatherers, Iron Age farmers and even Kraals have been discovered here.
Pro tip: check your maps before you leave as sometimes the traffic can be quite bad in that area, so you can save yourself some time.
Enjoy the views!