Could you describe your language learning background and/or learning style preferences? How long have you been studying Spanish? What kind of women do you like? I think that will likely be the ultimate answer to your question and keep you from getting bored.
Your English is very good. I would imagine that you would pick up Spanish much, much faster. EVERY Brazilian I've ever known can at least read most of the Spanish they come across and from that understand the vast majority of the pronunciation. I am fluent in Spanish (but not a native speaker), speak intermediate Italian, and can read a large amount of Portuguese. I have a hard time understanding most of it when spoken rapidly and maybe understand 20%-40% when spoken slowly and clearly.
Personally, I liked Bogotá when I was there but I haven't been for gaming. Geographically, it's well placed for weekend trips to other cities within the country or even another country if you get bored with Bogotá. That said, it is somewhat elevated so it can get a little cold at night and early morning.
When I saw your thread title, my gut was to tell you Medellín as well, but I haven't yet been there myself. I think the ROI in Colombia is decent enough to heavily focus there. There are many cities there. The women will keep you interested.
If you are going in April/May, I'm sure being Brazilian you know that school is already back in session and you are going into winter as June/July approaches. It will be cooler. Coastal towns will likely be warmer being closer to the beach (and thus sea level, not in the mountains) but it's also during the school year and the younger women would likely be attending school.
So, I highly recommend looking into the top 10 university towns in your countries of choice and going from there. I'm sure you could find a relatively inexpensive private tutor majoring in Spanish language, literature, or linguistics who could help you out. This should be cheaper than just going to a language school but to each his own.
In the event you choose Argentina, I would agree with Roosh that Córdoba, Argentina would be a better move than Buenos Aires, which is amazing, but more expensive and seems much harder to game. Chile would be easier than Argentina, but I feel the ROI is lower than with both Colombia and Argentina.
Chile as well as Argentina and Uruguay have very distinct accents. So, does Mexico in its cadence and more. I find Mexican slang and other vocabulary to vary a lot across the country but the accent not as much. You will walk away understanding rapidfire Spanish that 100+ million people speak, but I do find the Colombian accent incredibly clear and mostly neutral. So, in Medellín you will walk away with a relatively clear accent and likely plenty of bangs in the 7-7.5 region or hopefully higher.
Just having a girlfriend (or few) over 6 months and banging her would teach you Spanish while allowing you to skip paying for private tutoring except for the more advanced topics that you'd like to discuss. I would put the money into good logistics wherever you go. Something tells me you'll not be spending too much time in the classroom and much more time in the bedroom.
Take a look at these (
freely and publically available) documents from Rosetta Stone:
http://resources.rosettastone.com/suppor...rkbook.pdf
http://resources.rosettastone.com/suppor...rkbook.pdf
http://resources.rosettastone.com/suppor...rkbook.pdf
http://resources.rosettastone.com/suppor...rkbook.pdf
http://resources.rosettastone.com/suppor...rkbook.pdf
I would read them and go through them to see where your sticking points are and then hire a tutor to help explain the grammar and maybe practice a little verbally. So, again, go where you can afford to be around the women you like. Please let me know what you think.
Regarding your "quite good looking" 19-year-old self... you should fit right in at the universities, even if you only study at a private language school a few hours per week and just hang out on campus.
If asked by one of the girls there, just say you are studying abroad and have to take some preliminary Spanish language classes to get into the universities there and you wanted to visit the universities first and see for yourself which ones you want to go to.
Maybe you can get one of them to go on a little instadate and show you around campus. Maybe she'll show you her dorm and you can go from there.
If you change your mind about the cold or have a desire to go skiing in South America, I can ramble about where to do that.