My two cents from a gringo's viewpoint: Here are the places I've traveled, in my order of preference. I'm not completely fluent in Spanish, but I speak it fairly well, which is obviously very advantageous in the less touristy areas. I have never felt the least bit threatened in anywhere I've traveled, even when lost by myself in the dark in the middle of nowhere. That's not to say that you shouldn't take some precautions and be wary, but in my opinion I would be far more worried about a traffic accident than being the victim of violent crime. A few people come to harm every year, but out of the millions travelling, the risk is pretty low. In Latin America, nearly all of the culturally and historically interesting sites are tainted in some way by the rampant slaughter and subjugation of the native people by European colonists, which always makes me feel sad and guilty. That doesn't make them less interesting, though.
1) Medellin Colombia - Medellin has made a complete turnaround from the dark days when it was the murder capital of the world. I'm particularly impressed by the pervasive and spotlessly clean metro system, complete with aerial gondolas up the hillsides. The Flower Festival in August is a unique cultural experience, and in December they light up the entire riverfront with glowing sculptures. The people are very ambitious, friendly, and generous. Even wandering through the slummier sections, I noticed kids playing soccer and chess (there are sections where you should definitely not go, however). You are a short jaunt away from a tour through the coffee country. Only a 3.5 hour flight from Miami.
2) Costa Rica (Pacific Coast) - If you want a vacation on the beach and/or in the rain forest, then this is the easiest destination. Most places are pretty touristy, and it's not as cheap as other Latin American destinations. Pretty much any location outside of San Jose can be recommended.
3) Bolivia (La Paz and altiplano) - The least touristy, and therefore most authentic (some would say), experience. One of the few train rides you can take in South America, from Oruro south into Argentina, it crosses part of the salt flats full of alpacas and flocks of flamingos. Stop at Uyuni to venture into the largest salt flats in the world on one side, or the Andes on the other. Climb the easiest 6000m peak in the world (not that easy, but definitely possible if you're in reasonable physical condition). Take a tour of the hellish conditions in the Potosi mines, the source of much of Spain's wealth (first, make sure to buy some coca and dynamite, easily available on the streetcorner, to offer as gifts to the current miners). Not many people speak any English outside of the few tourist zones, and some don't speak Spanish either! Aymara is commonly spoken (I didn't learn any). I believe a visa is still required for U.S. citizens.
4) Cartagena/Santa Marta Colombia - The beaches are okay, but full of people and pushy vendors. Instead, take a snorkeling tour, which I thought was much better. Definitely some good history to be seen here.
5) Mexico City - Tons of history and cultural interest here. It has a very European vibe, lots of diverse restaurants and people walking everywhere. Think of the stereotype of the sombrero-wearing Mexican, taking a siesta in the shade of a cactus, and that's the exact opposite of what you'll find here.
6) Mexico (Gulf of California coast) - Much closer to that Mexican stereotype. I've been working for the last six months in a sleepy little town on the mainland side of the Gulf of California. It's a good place to relax, sip a margarita on the beach, do some kayaking, mountain biking, what have you. The Baja peninsula shields the ocean on this side from strong tides and waves. I haven't got around yet to taking the ferry across to Baja and doing a road trip. Warning: Summer temperatures routinely climb above 120°F. October through May is a better time to visit.
7) Bogota Colombia - Some very interesting sites within half a day's drive. I recommend visiting Andres Carne de Res for a steak. Even though it's not as big as Mexico City, from the top of Montserrat it seems to stretch out forever, just unimaginably huge.
8) Asuncion Paraguay - Not much of cultural or historical interest here. Flat, ugly, and boring. That sounds harsh, I'm sure it's not a bad place to live, but if you're going on vacation, you can pick someplace more worthwhile.