I keep extra cash in Vanguard VMFXX which is earning over 5% (5.27%) right now and will continue to with rates where they are. I have my regular bank accounts linked to it so I can move money back and forth, which alleviates any concern about check writing.
https://investor.vanguard.com/invest.../profile/vmfxx
If you use Apple, their account even earns 4.35% now.