How to travel for free or cheap money?

I’m sort of doing it now. Not with zero money, but with very little amounts - and often spending no money at all.

I mostly hitchhike, so I don’t pay for public transport (exceptions being usually buses and trains that get me in and out of big cities; when I avoid big cities, I don’t need these at all. Even with big cities, I sometimes walk out by foot and hitchhike). It is possible to hitchhike boats (and even planes) and I plan to do it - so far I have only hitchhiked cars, trucks, and the ferry to the UK (or rather hitchhiked a car that was going on the ferry, since the payment is per car and not per passenger).
I use Workaway - a platform that connects people who need some kind of help with people who want to travel and help. This includes accommodation, and often meals as well.
I Couchsurf. Couchsurfing in particular is not something I would approach with zero money - it’s an exchange network and as a surfer I want to contribute, not just take. It does save accommodation money and occasionally food money as well, but I never expect my host to provide me with food and even if they do, I try to buy some provisions myself or bring them something else that they might enjoy.
I pet-sit, which means I take care of pets in exchange for accommodation (and sometimes food as well, but again, it’s not something I expect).
I camp outside.
I occasionally stay with friends, family, or new people I meet who kindly agree to host me.