If it is about "g", general intelligence and reasoning power then I say an environment that encourages and satisfies curiosity to as large a degree as possible. You want rich and varied content with the ability to test hypotheses. It could just be a patient parent or ton of manipulatives or the internet or whatnot. But if a mind is "turned on" to questing for knowledge and deeper understanding, then tada.
From my point of view, there are some variations in brain wiring that are going to respond better to different types of environments. This essential factor is the basic size and distribution of minicolumns in the cortex. At one end you have larger minicolumns (i.e. more neurons per) that are widely distributed, at the other end you have smaller minicolumns closer together. As the further ends of this distribution correlate to Dyslexia and Autism respectively, they seem to be a source (i.e. largely genetic) contributor.
The larger mincolumns seem to favor breadth of experience and long connections to abstract concepts; sometimes referred to as inductive learning. The smaller minicolumns seem to favor narrower depth focus.
