First, check your broker’s cash sweep options. They typically offer a small interest rate with little risk. As for ETFs, SHV appears to be the closest thing to a free lunch available. It has had practically zero drawdown since its inception in 2007. https://stockcharts.com/freecharts/perf.php?SHV
GSY, MINT and NEAR normally offer better yields, but they do hold corporate bonds, which have dropped in value, and are they are currently trading at a discount to their NAV.