I wish I could give you a more straightforward answer but I unfortunately can’t. As you already know, there is no such thing as a free lunch (or is there?) so you will have to work really hard to get an opportunity like that. As usual, it is easier to get discounted prices on your travel cost, like, a free night stay in a hotel or a free tour in the city you visit. If you’re trying to follow that path, you can take baby steps like volunteering abroad or being a travel writer but here are some other options you can look into.