A lot of those "checks" are just things that Presidents historically don't do, not that they're prohibited from doing so by the Constitution. Even then, it's not clear that Trump really gives a shit about the Constitution. Like all contemporary Presidents he would decide what he wants to do first and then have his lawyers provide some veneer of constitutionality to it, but in Trump's case the bar for such would probably be really, really low.

He's already mentioned going after news outlets that criticize him. What else can the President do? Can he instruct the military/NSA to spy on every sitting Congressman and Senator, and blackmail them? That's not explicitly unconstitutional, and these days "explicitly unconstitutional" seems to be the only thing that matters (i.e. unless the Constitution actually says you can't do a thing, then you can do that thing). What are the President's actual powers during a time of war, especially if the theater is in America itself? Can he mobilize the military in response to a domestic terrorist attack, and if so what sort of plenary power does he have domestically?

And the more important question, really, is not "what does the Constitution say" but rather "what are the American people ready for"? If Trump does become President, the answer would seem to be "quite a lot of really scary shit".