Pay Down Your Mortgage
This is probably the lowest hanging fruit in your investment choices.
Although we often hear that mortgage is tax deductible so it's worth to keep even during your retirement. However, interest earned on money not used to pay down mortgage is taxable. Furthermore, if you have safe money invested in CD earning 2%, it's better to use that money to pay down mortgage that costs you 4%. If you need liquidity, open a home equity line of credit which won't cost you anything if you don't use the funds.