1. Volunteer programs
Let's stipulate right away - it's not exactly free. The budget for such a trip depends directly on where you want to go. In most cases you will need to pay for transportation to the project location, membership fee, personal expenses and, if necessary, visa. Accommodation and meals are covered by the organizers. In return, you'll be cheerfully rebuilding disaster-stricken sites, helping local communities in the Chinese countryside, or running a turtle farm in Sri Lanka - there are thousands of volunteer opportunities. You can take your pick from one of the volunteer organization websites:
If you're new to volunteering, your best bet is to contact a local organization that works with international volunteer projects. They will make sure that you are accepted into the chosen project and will take care of any issues that may arise. For example, in Kiev, the organization "Alternativa-V" does this.