Now, if you're asking is it legal for the POTUS to take executive action that essentially mimics legislation being proposed in the congress or that essentially directs Executive administrations to disregard laws by the congress, then the question is a big resounding "It depends".
SCOTUS Justice Robert Jackson is the person credited with laying the foundation for the legality of executive orders, noting there are three types that exist. Specifically, one type that is likely legal, one type that is likely illegal, and one type that is rather borderline.
It is clearly legal when persuant to an expressed or implied authorization by congress. It's likely illegal when the actions are incompatable wit hthe express or implied will of hte Congress. Then there is what's known as a "zone of twilight" where the President is acting on something Congress has seemingly not spoken on either way.
So taking something like drug reform, that another poster threw out there...
If it seemed that in a general sense the Congress was likely going to be taking some action on drug reform and it's reasonable to believe it will occur, it just depends how long the debate and process may take, the President would likely have significant latitude to take executive orders as it relates to drug reform that is in line with what seems likely to pass congress while he waits for an actual bill.
If it seemed that in a general sense the Congress was unlikely to be taking certain action on drug reform and it's reasonable to believe it won't occur or occur anytime in the relatively near future, then the President likely has very little legal latitude to take use an executive order as it relates to drug reform that is in line with the what seems likely to fail to pass congress.
If congress simply wasn't dealing with drug reform and it doesn't seem to be on their radar anytime soon but the President felt some kind of loosening action was needed, then he'd likely have a legitimate argument for taking action with an Executive Order in a legal fashion. However, it would depend on the extent of what he did and whether or not Congress then decides to take up the issue.