Which country is a great place to retire young to for an American where the USD goes further? South Africa? Cape Town? Costa Rica?

It'll be a place that actually allows that kind of immigration (because generally one must work and pay taxes to be able to move to a new country). Mexico allows a bit of this by Americans (who can lease but not own land there). Costa Rica generally doesn't and the retired Americans I know who own property there can only visit for six months a year. So do some research on what your actual options might be.

Spain.

That might depend on what you are looking for. South Africa has some serious issues right now: investigate. Do you value having local currency that is tied to the U.S. dollar? What weather do you like? What activities will you do? How will you get healthcare? Do you value easy/cheap returns to the U.S.? Have you investigated immigration rules? Very few countries will let you move in freely. Can you prove an income? I've lived in 3 countries.

Cape Town isn't a country.