Hi IrishGunner,
I really recommend Bolivia. The scenary there is the most amazing I have ever seen in my life. In particular I recommend the south of the country - the Salar de Uyuni. It's an enormous salt desert surrounded by snow topped volcanoes, lakes of different colours, really unusual rock formations, geysers, etc. You can spend a few days in a jeep travelling around there.
For recommendations on where to go, it depends what type of holiday you want. For example, Chili and Argentina feel much more European than Peru, Ecuador, Bolivia, and so are much less of a culture shock.
A sample route I'd recommend would be - fly to Peru, travel overland to Bolivia, down into Chili, fly over to Buenos Aires and up towards Brazil...