I've lived in SE Asia, been all over there, travelled through Europe, and travelled through South America. I personally really enjoyed Peru and Colombia. My buddy loved Brazil and my gf is in love with Argentina. I've found its usually not the place, but the people you meet and/or the experiences you have.
If you speak spanish, you have a leg up and could have a great time anywhere in South America. I would avoid Venezuala, and personally didn't like Bolivia much but find out on your own. Depending on how long you want to go, start in Colombia, bus to Peru, Brazil (if you want to get the Visa, otherwise not necessary), Argentina, Chile, etc. Don't have set plans, if you find a place you like stick around for a week or so. If you don't like it, move to the next.
Other alternative is to pick a place to settle down in for a month+ and learn a skill. My personal ideas would be salsa dancing in Colombia, surfing in Chile/Nicaragua/anywhere in Latin America, tango in Argentina, jiu-jitsu in Brazil (my buddy has been doing that for 9 months), Spanish guitar in Mexico... whatever you want. I personally don't like just hanging out and if you have something you're doing (surfing, yoga, jiu-jitsu, dance, lifting... whatever) you will meet people doing the same thing and will have a good group to hang out with.
If you're thinking of doing it, do it while you can. Roaming around another country can't happen once you have stronger commitments at home. Read the book vagabonding, if that doesn't connect with you then you're not meant for backpacking.