You can go further and buy government bonds directly, through the Tesouro Direto system. Those indexed by the SELIC rate (which is sort of related to the CDI) are currently around 13.5% per year, before administration fees and taxes (in fact, the boring funds I mentioned above mostly invest in these SELIC-indexed government bonds).
This will change when (and if) rates get lower, but so far, investing in these funds or bonds is the simplest (and one of the best) way to have passive income.