Read up on the issue of flying into Costa Rica on a OneWay, as the airline is happy to sell it to you, but you will need proof of Leaving the country within 90 days of arrival, before you check in for the flight to CR...the "Onward Travel" requirements varies by airline, passport and border.
The proof needs to show you are leaving Costa Rica, not another country.
Adventure Travel to Colombia, Costa Rica, Nicaragua, Honduras, Guatemala, Mexico, Thailand, Burma, Laos, Cambodia, Vietnam, China, South Africa, Morocco, Turkey, EU, USA National Parks, enjoying culture, cuisine, motorcycling, scuba diving, surfing, sailing, rafting, hiking, fishing, camping, nature, wildlife. Get a Guidebook, and get lost!