It is relatively easy to convert a tourist visa to a work permit without leaving the country. Most travellers wanting work find it in the various English language schools, who are always seeking native speakers to teach. The pay will not be good, but enough to get by on. Unemployment is fairly high, so finding work elsewhere is unlikely unless your Spanish is really up to scratch. For an overview of what's available, check out 4icj.com. There are occasionally volunteering opportunities available, although they are less common than elsewhere on the continent.