What to Pack
What you take on this trip really does depend on how active you are going to be and which parts of the country you will be visiting.
Costa Rica is a tropical country, so it can get hot during the day, with mild evenings.
In the east and south-west of the country (Pacuare River, Tortuguero, Osa Peninsula) you are likely to experience hot, humid days with some tropical downpours.
In the Central Valley (including San José) and Arenal the weather is warm and pleasant.
In the cloudforest (Monteverde, San Gerardo de Dota) it can get quite chilly, especially at night, and the air will feel damp.
On the Pacific coast (Nosara, Samara, Manuel Antonio) days will generally be clear, sunny and hot.
The northwest (Rincon de la Vieja) is very hot and dry.
The best advice is therefore to bring light, cotton-clothing that will help keep you cool and bring a waterproof with you for the jungle and the cloudforest.
A fleece for cooler evenings in the cloudforest is also a good idea.
For jungle walks your feet should be well protected with good ankle support and effective waterproofing.
The best policy is to bring equipment you already own and are comfortable with.