One of the Bitcoin relatively safest and passive income is from Bitfinex margin funding program. However, the rate fluctuate daily. So, you might want to get into 30 days funding if you think the rate is good, and shorter days if you think the rate is too low.
How do you know if the rate is good or too low? I visit BfxData.com. For example, historically the rate reach above 0,04% per day. So, if you see that on offer and you have funds available, you might want to grab on it for 30 days. Otherwise, grab on whatever you can for 5 days, and repeat the process later.
The blue thread in the middle is the volume weighted average rate (Vwar) at around 0,02% daily (or 7,3% annualy). This is much higher than 1% annual saving interest rate from US Banks. See, the usd.deposits.org as comparison.