Take a good Spanish textbook or basic text with you, preferably one that you are comfortable with and have learned from.
Of course, take a good Spanish/English dictionary with you. We have found that the Merriam-Webster Spanish-English Dictionary is a good resource. It concentrates on Latin American Spanish. It is only $5.95 US.
Study as much as you can before you go. This will help you get even further when you arrive at class.
Take a small container with office supplies such as, index cards,rubber bands (to go around your note cards) notebook paper, paper clips, pens, pencils (a sharpener), White-Out (for mistakes in your notes, scissors, and possibly even a stapler. We also found a 3-ring notebook to be useful for organizing our many notes. A note on these office supplies: We bought all this material in Heredia, across the street from the Central Park. These cost a bit more than they do in the U.S. but buying these items in Heredia was a pleasant part of our education.
Price some of the weekend tours you might want to take. They can cost a good bit of money and many times they do not take credit cards. This may leave you cash poor in Costa Rica.
Take a few bottles (pocket sized) waterless hand sanitizer with you.
The corn flour sacks available at the Central Market (1 Colon or 35 Cents US) are great ways to carry things you need. I wish I had brought more of them back to the U.S.
Toiletries are a bit pricey, so take those that you use.