1/ Check your Withholdings and Quarterly Payments
In order to avoid underpayment penalties, check your 2019 withholdings and payments made during the year. Both at the federal and state level.
2/ Tax Status for 2019
You filed 2018 either as single taxpayer, married filing a joint return, or married filing a separate return. If your situation changed during the year 2019, or if you are thinking about changing your status early 2020, it is still time to take action before the end of 2019 to see what will be the optimum status and best fit for your tax situation.
If you donated to charities, it is still time to send your gifts before 12/31! Also understand that you will be able to take a deduction if your total itemized deductions are above a certain threshold. An estimation can be made.
4/ Foreign Properties
If you owned foreign properties in 2019, you might want to make sure you optimized your assets. Also, if you owned foreign bank accounts in 2019, you might need to be compliant with the regulations and report such accounts.
5/ Business Owners
It is this time of the year to foresee what will be your tax liability for 2019 and make sure you are compliant with laws and regulations.