Good points, but the OP seems a reasonable, responsible sort who has stuck by her original desire to avoid taking out credit cards to get the miles in response to a number of suggestions. I think of this like a high school or college graduation trip--a reward well earned for the hard work required by medical school and a needed break before jumping into the even more intense work required in hospitals. All work and no play. . .

Nevertheless, OP, you should be sure you have all of the likely costs estimated before you go. Airfares, though high, may not equal the amount you'll pay once you're on the ground in Costa Rica for lodging, meals, and transportation. You can't predict everything, but you won't want to do all of this hard work in accumulating miles and points simply to realize that you'll still need to spend another $2000 to make your trip come true.

Fortunately, some of the information may be found by doing Google searches around the web as well as by looking at another forum here on FT dedicated to travel in Central America: http://www.flyertalk.com/forum/central-america-479/.