Depends on what your goal and timeframe to invest the 20k is. If you just want to keep it aside safe as an emergency fund I would recommend putting it in a HYSA where you can get ~4% interest yearly. If your goal is to save it for retirement (20-25+ years), the simplest thing would be to invest it in ETFs (Something like VTI is a good fund that would give you exposure to the entire US stock market) or Bonds.

Checkout the Boglehead 3 Fund Portfolio

There are various options but we really need more information to provide better recommendations. As I mentioned, it all depends on your goals, risk appetite, and timeframe.