He will bomb Iran on his way out. Wouldn't be surprised he does when all legal avenues for him to steal the election is exhausted. It will leave another major mess for Biden to clean up. Domestically there is little he can do. He can't stop the electors on Monday and he cant stop Congress on January 6th. He could try to invoke the insurrection act but he tried to do that this summer and it didnt succeed. Internationally is where he could do damage. And bombing Iran is the one thing he may do to make it very hard for Biden to return to the Iran nuclear deal, make Biden deal with the geopolitical, economical and military disaster that would come form this not to mention dealing with Covid-19 plus an economy on free fall.
I definitely see him fukking up shyt in Iran, but I can also think of three major things he can do domestically:
#1. Somehow fukk up the vaccine rollout or otherwise make a total mess of the coronavirus response (delaying supplies, firing everyone in the CDC, etc.), so that it becomes very difficult for Biden to start his own shyt effectively and the winter wave accelerates into Biden's term.
#2. Totally screw up staffing across the entire federal government by firing everyone he can and installing his own lackeys everywhere he can, even in positions that aren't supposed to be politically appointed.
#3. Straight destroy or lose important records or make them difficult to fine or make sense of, so that the Biden administration is delayed in significant ways
None of this has really been on the table before in a presidential transition, but no one like Trump has ever been president before.