There's really nothing that you can do to earn without investment. Time and effort, themselves, are investment. But if you're pertaining to money investments, then there are ways on how you could do that — with the help of your skills and services that you could offer.
Can you write a blog? Do you know programming? Things like that. If you want to earn Bitcoin, those jobs are usually the sources (also look in the services section). But if you just want to earn money (not necessarily bitcoin) then there are also jobs (can be online or not) out there that you can do. Just know first what you're good at, or what you're capable of doing.
Maybe look into doing some online surveys? They are usually being paid for a small amount.
If you, op, are willing to invest your time and effort in surveys, then go ahead. That's also a good source of income, though it does not pay that much and sometimes, a survey doesn't fit you (I experienced that as well). Best thing to do is work using your skills and knowledge. It's more fulfilling that way.