So far we can show that intelligent enhancement can be predicated on exercise, outside of that there are temporary drugs (e.g. coffee) that "boost" intelligence but in actuality boost attention.
From Paul Tough's book "how children succeed"
Forget the M&M's. Video games are a skinner box on steroids. Kids are going to learn fast.
Edit: I may have missed the point -- it looks like motivation improves performance, not necessarily actual "cognitive ability", if we can quantify that.
Medications that address such things this may not "improve intelligence", but at the end of the day, if they let someone concentrate and be productive for 18 hours in a day, with no "hangover" the next, or lets them visualize complex protein makeup while altered, with enough retained to write up a scientific paper about said protein when sober, isn't that close enough for the pop-sci definition of "improved intelligence"? What the person is able to do with them, is still better than if they did not. Nootropics and micro-dosing are popular topics for those trying to use medication to improve overall effectiveness, in the quest for the drug that's still science fiction featured in the movie Limitless.