Low dose ketamine has been shown to promote synaptogenesis in the prefrontal cortex. (in rats) Link to abstract
It is currently being investigated as a potential antidepressant in humans, but based on anecdotal evidence, it seems likely that it’s also a nootropic.
Alexander Grothendieck used the analogy of opening a nut to illuminate two different styles of doing mathematics. One way is to strike the nut repeatedly with a hammer and chisel.