Central America is popular amongst career breakers wanting to do volunteer work - and those who just want to travel.
Central America covers the following countries (working from north to south):
The region borders Mexico in the north (which is counted as part of North America) and Colombia to the south. Over to the east are the Caribbean islands.
Central America has lots of opportunities for career breakers. There's a wide range of volunteering projects there - from teaching and community work, to conservation (both marine conservation and rainforest). A number of language schools also operate there, teaching Spanish. If you just want to travel, with a group or independently, Central America has a lot to see. From ancient Mayan ruins to lively cities, beaches to rainforests to volcanoes, this diverse region offers a lot to the career breaker who's looking for something different.
Lots of people fly into Mexico City and work their way south into Central America. You can also fly into one of the Central American capital cities (like Belize or Guatamala City) either directly or changing in the US. If your round-the-world ticket includes a stop in Los Angeles (which many do), you can travel overland through Mexico to Central America.
The 'when' depends on which bit of Central America you're going to, as the climate is quite varied. You can go there any time of year, but if you're planning to go somewhere remote, be aware that some roads might be impassable during heavy rainfall, so you may wish to visit outside the rainy season (which varies from country to country). Visiting outside the peak season means you'll be faced with fewer crowds, plus you'll be supporting local businesses at a slow time of year.
Central America is a relatively poor region. If you're not going there on a volunteer project, at least make sure you travel responsibly. Buy from local businesses, pay attention to instructions from your host, guide or guidebook, and be thoughtful about the impact you're making on the country and the people you're visiting.