The Namib dune sea?
Table Mountain would have been my first choice (of course), followed by Kilimanjaro, Vic Falls, the Sphinx, statues of Ramses II, Timbuktu as a whole city. Oh, and the Big Hole in Kimberly.
Carnac, if you think the Richtersveld looks anything like the Serengeti, you’ve never seen the Richtersveld. Hardly a blade of grass for miles and miles. Lots of rocks.
What’s a train doing cross Niagara Falls?
That would be: What’s a train doing crossing Niagara Falls … cough …
Where the Wild Welwitschia grows.
If you see one of those popping out of the ground, you can be pretty sure you’re in Africa.
Ask the OP. I wouldn’t recognize Kilimanjaro, and the Serengeti was my idea. 
Sub-saharan Africa is more known for its wildlife than its human made structures. However, I would think that many educated people would be able to identify Great Zimbabwe.
Does the Sahara count?
I think Mr. Dibble provided an excellent answer, since you were looking for something along the lines of the Pyramids: The Sphinx.
So, when there aren’t any snows on Kilimanjaro anymore, do we cross one off? (I’m not sure I’d recognize it as easily snowless.)
Someone’s mentioned the mosque in Timbuktu-- I’d say just about any of those cool mud mosques in Mali. Love those.
I would say there a far more memorable man-made landmarks in Africa than North America. Off the top of my head:
To man-made, I would add the Ramses statues at Abu Simbel, the Temple of Karnak in Luxor, and the great mosque at Djenne, Mali.
