Hi BraveWings and Welcome!
You've already had lots of good advice, now just to add my two cents

While the West Coast can have some wet days in September, most days will be mostly sunny and comfortable and WCNP is lovely — and not only for the flowers in the Postberg section. Don't skip the rest of the park, which also has plenty of flowers and other interesting plants, animals and birds. My favourite spot is Tsaarsbank and the hide at Abrhamskraal can also be rewarding. As well, the whole area around Darling has several worthwhile wildflower reservations and Darling Hills Road (which also boasts the Groote Post wine farm) is a favourite drive for me.
Closer to Cape Town there are good hikes (I'm told, not a hiker myself) in the Silvermine section of Table Mountain — your SO could do a hike while you enjoy just spending time at the Silvermine reservoir, a nice stroll around the res and several well-placed benches along the way to enjoy the views and the birdlife. And there's lots more to Cape Point than the point itself — I recommend the drive to Olifansbos and the road down to Black Rocks, from where you can walk to Venus Pool.
Venturing farther afield, you might consider doing a circular route visiting several parks: starting from Cape Town and head first to Karoo NP, then to Moutain Zebra NP and from there you can choose to visit either (or both) Addo or/and Tsitsikamma (and perhaps Bontebok) en route back to Cape Town.
Enjoy your planning and your trip!
PS: Okie's thread is in the Chit Chat section