I have been trying to earn bitcoin without investing for months already. And i tried to earn through signature campaigns. There are of course other ways to earn by mining and staking coins. However, for staking coins you must have ways to get them first before you can earn more. As for mining, cpu mining is hardly possible for most of the POW coins now. And it is even meaningless as the gain can hardly offset the electricity.
There are a lot of things someone can do to earn bitcoin without investment in this forum alone. Just in the marketplace in this forum, you can find ways to earn like selling some goods for bitcoin or do services for others like joining signature and social media campaigns. If you have some skills then maybe find a job that is looking for someone like you that possess those skills. It's not money invested here but time and then eventually when you earn you can start investing or trading which can help you earn more.
Earning bitcoin through faucets may be an option as well but it will take time to accumulate and they are giving only small amount but if you have so much free time then you can try it and then build up your referrals so that you can still earn even if you don't spend much time on it. It would require hard work at first but eventually, if you have already established something then you just have to maintain it.