First of all, go to this free webpage and print it. It's a very complete list of Costa Rican Spanish words and phrases, including words for foods most commonly enjoyed in C.R.:
http://www.paulglassman.com/cr14.htm
Second, and this is important: In costa Rica, the informal "tu" form is rarely used. Nicas and Panamanians, Colombianas will not be offended by the less formal form of address, but the ticas might take some offense.
Es muy linda (You are very lovely) is preferred over "Eres muy linda."
Now for a few dirty words
panocha (pah-NOH-cha) = the female organ
picha (PEE-cha) = the male organ
Es muy linda, pero no puedo pagar cien = You are very lovely, but I can't pay (afford) so much.
Note that unlike English, vowels in Spanish only have one sound:
a = "ah"
e = "eh"
i = "ee" (as in "tree")
o = "oh" (as in, "oh say can you see...")
u = "oo" as in blue, fool, cool
Remember that the many inexpensive Spanish language schools found in Costa Rica aren't just great education in the language, they also make a great excuse to tell family/friends/gf/B&C. And for some, doctors, lawyers, cops, the trip may be tax-deductible. Consult an honest attorney.
Hope this helps. PM me with any other phrases you may need, and have fun in C.R.