Warning: a trip to South America will take you more than two years, and thats just seeing the highlights;)
but i would highly recommend these places-
1)Peru - Macchu Pichu. You could go there, spend a few days and return without doing anything else. Its that amazing.
2)Venezuela - Merida and Roraima. Merida is an andean town, university town filled with the hottest girls i've ever seen in one place. The town is beautiful, nestled in the andean mountains.
Roraima is a Tepui mountain where from Conan Doyle based his novel The Lost World.
While its well worth getting to the top (about 9000 feet) don't do it unless your physically fit or you will regret it. Takes about 5-6 days there and back but its incredible.
Also the caribbean coast of Venezuela is fantastic, very inexpensive and the scuba/snorkel tours of out of this world.
3)Colombia - ;)
J/K some great places here. Cartagena on the coast, Medellin inland, Bogota, and my favorite San Agustin in the south and Leticia which is an amazon border town.
4)Brasil - take boat from Manaus, Brasil to Leticia, Colombia. Spend extra money on cabin, or scrap boat trip, see sights in Manaus and fly to Leticia, Colombia where the jungle tours are awesome and you can catch a fast boat to Iquitos, Peru which is freaking great. Its the birthplace Of Fitzcarrldo, the famous rubber baron and basis of the Herzog movie. Great bar right on the river with a very hot bartender. She speaks Portuguese
