I'd go to Peru. Lots of available women (Lima is a big city and has lots of hot women to choose from), great jungle experiences (Amazon), mountains (around Cusco), and airfare isn;t too bad getting there. You can surf or handglide right in Lima if ya want. There is white water rafting and all the standard stuff there too. There are enough tourists that lots of facilities exist on the gringo trail yet alot of unchartered terrority still exists too if you want that experience.
Argentina is more of a refined place, Buenos aires is a classy city and the mountains in the country are nice. Very nice country all around.
Brasil requires a visa for US tourists (not sure when you're leaving or what country you're from, so this may or may not be an issue other than a lil extra cash).
Costa Rica has too many gringos and is not exactly an authentic Latin American experience, but they do have a lot of tourist facilities there (which you'll pay through the nose for if you don't pay attention to what you're doing and who you're doing business with). Because of this everywhere in this country has lots of tours and such setup, so it's very easy to do things here and get around but there are few areas where you'll be a pioneer in the tourism trail here.
Gutamala -- Sorry about the spelling but I cannot imagine the women there are very attractive. Probably would bring my own for this place or Belize. I'm going to Belize later this year but for the reef not the women.
Nicas are not bad, but some parts of the country are getting touristed now. I'd opt for the untouristed domains and blaze my own path, but don't expect everything to be all rosey, you may have some rustic accomodations in rural areas.
Really it just depends, do you want to follow the tourist trail where you're just another face in the crowd or do you want to standout as the only gringo in town. You can really be either of those things in Latin America. Do you want first class accomodations or can you withstand rustic? Cities generally offer a wider range of accomodations than some remote place. Do you want a beach all to your self or a party beach? Do you want to party with locals or with tourists? Do you want to see how peasants live or how the rich people live down there? Do you want hookers , swingers, or to run game with civilians. Any of those things are possible.
