I use these sites/apps basically every day when travelling and these seemed to work the best for my travel needs and for my backpackers mind to explore as much as possible while staying safe and on budget!
This site will make your volunteering journey a breeze! You can volunteer basically anywhere in the world, doing what you love and you can even get free accommodation and/or meals in exchange! I have been volunteering to teach English in Rio de Janeiro and that experience was definitely a highlight of my backpacking "career".
Worldpackers: Work Exchange, Volunteer Abroad, Gap Year