Rand could actually do more than that as President. He could simply direct his Attorney General to remove drugs from the Controlled Substances Act, which would legalize them at the Federal level, and leave it up to the states to legalize, decriminalize, and/or regulate however they see fit. (It's a common misconception that the President doesn't have the power to end the Federal war on drugs any time he wants. He DOES actually have that power, and not just by stopping enforcement of the law.)
The Controlled Substances Act is somewhat unusual as Federal laws go, in that
it does not require the action of Congress to change it, because of the way it's written. Section 811 of the CSA gives the Attorney General the power to make any changes he wants to the schedules of controlled substances. I think the intention was to make it easy to add new drugs to the CSA as they are invented or discovered. But it could equally be used to remove substances from its control. Again, with no action needed from Congress.
The Executive branch has the power to actually legislate in the Federal war on drugs!! They do have to follow some procedures in order to do so, but that's a formality that wouldn't stop any President or AG from doing whatever they want with this.
You can read more about it here:
http://www.theweedblog.com/attorney-general-holder-admits-obama-misled-on-medical-marijuana/
(There is a link within the article that takes you to the actual Section 811 of the CSA on the DEA/DOJ website as well, for those interested to read the whole section.)