hi lacie
7 days is indeed not a lot. it also depends on your interests of course. the garden route is very nice and definitely worth a visit, but the main reason for me (and many other people) to travel to south africa is to see wildlife, and for that krüger np is fantastic.
with 7 days, personally i would spend 4 days in and around cape town (including a days visit to the winelands). fly from cape town to krüger mpumalanga airport (there is one direct flight per day), spend 3 days in krüger np and fly to johannesburg to fly out of south africa.
between cape town and pe, its about 850km. you need 4 days (thats the absolute minimum in my opinion) for cape town and the cape peninsula (including a day in the winelands). which leaves 3 days for cpt to pe. if you dont want to go to krüger np, spending at least a day in addo elefant park (80km north of pe) is a must in my opinion. there you can see large herds of elefants and plenty of other wildlife. which leaves 1-2 days for 850km. which will mean a lot of driving, and not a lot of time to do / visit something on the way.
you could also easily spend 7 days in and around cape town and going on a 2 day excursion from there. if you travel soon (up to november), to the whale coast (hermanus / gaans baai / de kelders) to see whales (about 2 hours east of cape town, longer and more scenic if you travel along the coast). you could also travel up north to paternoster or langebaan to spend 1-2 days on the beach, go for walks, eat seafood. or up north to the cederberge for beautiful nature, bushmans painting, hiking, cycling etc.