I assume we're talking after weeks of court cases arguing over the 50 voters in fl/pa/mi/etc that ultimately decide the election - and that Trump has actually conceded...
Prior to him leaving office:
Probable:
- Pardons as many of his cronies as possible
- Finds ways to make it harder for Biden to undo some of his foreign policy moves - vis-a-vis Israel, Iran and China
- Tries to push through more confirmations of conservative judges as a lame duck
Plausible, but Highly Unlikely:
- Find a way to get Roe v. Wade overturned by SCOTUS before Biden gets a chance to pack the court.
- Take another shot at IRAN or China - diplomatically or militarily
After he leaves office:
Probable:
- Run for NYC Mayor in 21' or Governor in 22'
- I don't seem him running in 24' - He'l be 78 years old, and who knows where the country will be, but hey, he's trump.
- Spends most of his time in Florida
Less Likely:
- Goes to Jail
- Leaves the country permanently - to avoid prosecution or otherwise.