Kirstenbosch National Botanic Garden. Over 1 million visitors were logged here in 2014, but I can't find current figures. I do not imagine they have dropped. That puts this garden on a par with the botanic gardens of much more populous New York City. Plus, unlike at the NYBG and BBG, you may picnic anywhere on the grounds (this should be a democratic right). There are no trash cans, and I have never seen any trash.
Having the most beautiful garden in Africa on your Cape Town doorstep is not something I could ever take for granted.
I renewed my Botanical Society membership a few weeks ago and purchased a late Christmas present membership for Tipsy (plus guest, for a little more - and it also grants additional free access to a scholar between the ages of 6 and 18), when we visited last week. Membership fees gain you entrance to this garden - its stunning grounds, displays and exhibitions; give you access to all national botanic gardens across South Africa; and - most importantly - support the mission and work of the Botanical Society of South Africa, including projects like CREW.
At the nursery we also bought some Artemisia afra and Mentha longifolia for Tipsy's garden in Langa. Both plants are well known herbal remedies and if they survive the sandy soil and water restrictions, will be useful in her backyard medicine chest.
An hour at Kirstenbosch - watching birds, lying on the lawn, walking up the steep slopes, counting ericas, paddling in spring water, is an hour you do not need to spend on a therapist's couch. It may save some doctors' bills, too.