If you could be president for a day, what would you do that would positively effect others?

I don't think anything significant can be done in one day. But if it's just one action item you are looking for I think the best thing that can happen to America and the world would be to reduce defense spending by over 50%.

End the "war on drugs".

What about setting Washington on fire?

I would institute free dental for citizens of the US of A.

Surprisingly, I agree with that.
BUT, I think you mean it a different way then I do.
I would order every US DEA agent or any other US anti-drug personnel out of all foreign countries immediately.
Then I would issue a proclamation, warning the rest of the world that we would no longer try to stop them from shipping drugs.

What we would do is board EVERY vessel entering US waters, and any found to be carrying drugs would be sunk, on the spot.
The crews of those vessels would NOT be arrested, but they would not be rescued either.

Any small aircraft (the kind smugglers like to use) entering US airspace would be intercepted.
If the pilot refused to identify himself or otherwise ignored instructions, that aircraft would be immediately shot down.

I would end the war on drugs in other countries, and focus 100% of the anti-drug efforts on the US.

Your plan would create a lot of jobs too.

Now there's a well thought out plan. Kinda Browniesque in nature.