Luanda, Angola
I'd booked myself one extra day in Luanda (which sounds remarkably like "Rwanda" when the local people pronounce it, a fact that frequently confused the heck out of me when they were talking until I remembered the similarity).
One day turned out to be plenty of time to see the main sights of the city since there aren't very many. I'd still like to go out to see Angola's famous waterfall, which has the second biggest span in Africa, but this is about 5 hours dr...