Well, he has the nuclear codes, and at one point they were talking about an executive order (? or something) so that federal anti-discrimination laws no longer take "disproportionate impact" into account. That's a pretty big deal in the application of the law.
This one is an agency regulatory action in the works, not an executive order -- basically something that takes a lot longer and is somewhat easier to beat in court if improperly done, but also
much more substantive in effect (e.g. legally binding).
Now
normally an agency has to go through a notice & comment process in doing such a rulemaking, which requires publicly releasing a proposed form, taking comments (and potentially doing public hearings), then (theoretically) taking those into account while drafting up a final rule. No proposal has been published yet, so
normally this would mean there's no chance of them finalizing it before inauguration, which in turn means Biden can just toss it in the trashcan.
The Dept. of Justice is not doing things normally. The only form this publicly exists in is
esoteric meeting notices being held on "Amendment of Title VI Regulations," yet according to leaked text obtained by trade press the rule is going to be direct-to-final - skipping notice and comment completely, and being made effective immediately. This is... probably illegal, as while such exceptions are possible they don't really apply to this situation.
Separately, regarding the 25th amendment, on one of the talking heads segments of the news last night somebody raised the prospect of the 14th Amendment instead.
Section 3
No Person shall be a Senator or Representative in Congress, or elector of President and Vice-President, or hold any office, civil or military, under the United States, or under any State, who, having previously taken an oath, as a member of Congress, or as an officer of the United States, or as a member of any State legislature, or as an executive or judicial officer of any State, to support the Constitution of the United States, shall have engaged in insurrection or rebellion against the same, or given aid or comfort to the enemies thereof. But Congress may by a vote of two-thirds of each House, remove such disability.
. . .
Section 5
The Congress shall have the power to enforce, by appropriate legislation, the provisions of this article.
Now perhaps unsurprisingly in the post-Reconstruction / early Jim Crow era Congress passed laws
weakening enforcement of section 3, but the big difference here is the form of action we're talking about here - not 2/3 votes, but passing laws (i.e. majority house, 60 votes senate, and Biden's signature).
And, indeed, the one time I know of that it's been used - against an anti-war socialist during WW1 who was convicted under the rather notorious Espionage Act - after being elected, congress simply refused to seat him under grounds of the 14th Amendment. (A few years later he won election again, and seems to have been allowed then).
Now obviously it'd be harder for congress to block a president quite so casually as that given it's a different branch, but passing a new law may well work here. Given timing the main impact would be to block him from future office.
I wouldn't put it past him getting off a final parting shot against Iran (some remote/droney-thing, most definitely Not A War™, because he's not started any wars...) or some less obvious but equally uneasy action. Depending on several factors[1], it could get out of hand.
There was reporting a week or two after the election he wanted to attack Iran (not with nukes, mind) but was dissuaded by the military. I assume at this point they would either do the same or simply ignore him.
(Back in the Nixon era, according to later accounts of those close to him in the WH he would somewhat frequently get raging drunk and make orders that no sane person would follow - and so staff simply chose not to hear him.)