I was looking into volunteering in this animals centre near Puerto Viejo de Talamanca - www.jaguarrescue.foundation/volunteering/ . Beware that many places that have volunteers in CR have a fee that volunteers pay, sometimes called a donation, as well as have a minimum required term, and recommend to know at least some Spanish. There, the minimum is US$350. At least it was not so long ago. You also pay for own accommodations and food except for 1 week at La Cieba (their other location not that far, also on S. Caribbean). You can read the requirements for volunteering on their site. You would also need to have recent TB test results for working with primates (which you are not guaranteed to do, as it is a much desired task, used as a reward...).
Different volunteering opportunity in terms of tasks and area - http://www.proyectomontezuma.org .
Olivia, I'm a little confused by your post. Have you done any research on Costa Rica at all? I don't know how we can possibly help you decide where to go and what to do when we have no idea what you like, what your budget is (although I'm assuming it's low since your asking about Hostels) how you plan on getting around, how long you want to stay in an area before moving on, what type of activities you like, etc. Sounds like you're just going to 'wing it' and maybe you're just looking for ideas, but we need more info from you to best recommend something.