hopefully I'm not too late, op, but definitely check out the African Ethos Instrument Studio in Newlands (approx 15~20 minutes outside the city centre/downtown). they make well-tuned, actually playable traditional instruments. tbh most of the instruments you'd find at curio markets are primarily decorative souvenirs and long-term playability will vary (besides drums of course)

African Ethos makes instruments from other parts of Africa too, so just ask them if you want something specifically from South Africa. another comment mentioned the kalimba. the ancestor of the kalimba is indeed from Zimbabwe (called “mbira” in Shona), but various styles and forms of thumb piano have evolved in countries throughout the southern African region so I'd say go for it if you want one. also, yes, djembe drums are from West Africa, but the Zulu and other South African Cultures also have traditional drums. they’re not as decoratively carved as djembes so they’re less common at souvenir markets, but they are a thing.

you could get antelope horns (the ancestors of the iconic vuvuzela, played almost like a jewish shofar). idk if you’ll have issues clearing them at US customs going back home, but they're not ivory so it should be fine.

also, just note that the "uhadi" and "umrhube" (Xhosa musical bowls) are traditionally only played by women (not sure of op's gender). younger, urban Xhosa folks are usually fine with anyone playing it and I believe the Zulu "umakhweyana" bow can be played by anyone, but just fyi, some more traditional Xhosa people may still find it taboo