I haven't done it myself, but my friend lives in Costa Rica and travels to Belize. I could e-mail him and ask his suggestions. Depends on where you want to go in Costa Rica and your means of travel. Also depends on your interests and what you want to do when you are there. For instance, I assume you want to fly? If so, are you interested in the Pacific side (Liberia) or San Jose, the capitol? Do you want to spend time on the Pacific side or the Carribean? Don't know if you have been to Costa Rica before. I found it an awesome place, but wished I spoke better Spanish. It is inexpensive and the food is great! Plus, I have never seen such wildlife, rivers, jungles and waterfalls. Crime and theft are a major concern.
Did not make it to Pacific side, but highly recommend a visit to Fortuna (hotels are clean, safe and cheap), Arenal Volcano and Tabacon Hot Springs. I spent a majority of my 10 days in Puerto Viejo, Cocles and Manzanillo, on the Carribean side, near the Panamanian border. Pristine beaches, jungle, and no hotels on the beaches. I highly recommend the Lonely Planet travel guide. It is very accurate.