The president doesn't have near as much power as people believe. His main job is to enforce the law and put the brakes on congress with his veto power. He can influence all he wants and some of them are quite effective at it but still that's just influence.
Well TBH, the President can throw a giant wrench into the works through his control of the Federal Agencies.
The President, under current law, can:
1. Declare a given substance to be, or not to be, a drug - so he could legalize marijuana at executive fiat.
2. Remove weapons from the NFA's purview, so he can end gun control in America in a sweep of his pen.
3. Pardon people by the thousand.
4. Veto. Veto. Veto.
5. Similar to 1 and 2 with other regulatory agencies.