I would agree that seeing Havana before it falls down and gets redeveloped for Americans was something special.
For similar reasons I'd like to see China before it gets too westernised. If I'm not too late, that is. South Africa also sounds great, after from all the murders and muggings.
I'm lucky enough to have seen pyramids in Egypt and Mexico, and found it amazing that different civilisations so far apart basically had the same idea! In Mexico you can climb on them right up to the top, which I don't imagine will be the case for much longer.
My biggest recommendation though would be Sri Lanka, for the incredible culture, history, archaeology, geography, wildlife, food and people.