I don't think there is any convincing evidence that specific activities can improve the general cognitive functioning but its entirely possible to develop specific cognitive skills, with practice. You can certainly improve your IQ score quite significantly by practising answering the same sort of questions used in the test your going to take, its a bit like getting better at crosswords you get better with practice and better still if you practice crosswords from the same compiler.
This really represents changing the software rather than the hardware, I suspect attempting to alter the physical characteristics of specific areas may be a bit pointless, the Brain is a highly integrated organ that is perfectly capable of altering itself and it does. It shifts its resources, even changes the size of areas and their connections based on usage. The rules it uses to control this process are not really understood but it doesn't seem to alter its underpinning capabilities, but learning about a range of subjects can help maximise potential.
There has been a whole range of different drugs, used and abused in an effort to enhance cognitive abilities, these are collectively referred to as intelligence enhancing. These may be designed to effect neurotransmitters like acetylcholine which is involved in memory, health foods like Lecithin act as a precursor, there are a variety of drugs developed for Dementia's, most don't work by one called hydregen was popular, one off high doses of B vitamin's and of course amphetamines. These all come with their own problems as do all the electronic/magnetic devices sold with this effect in mind.
There is lots of stuff around wellness interventions like diet and exercise which are entirely credible but may be heavily influenced by the need to promote "healthy lifestyles". Few people consider that our bodies are quite capable of managing variations and the brain is a privileged organ in terms of the resources it gets. Its very difficult to show significant changes in cognitive ability from a healthier lifestyle compared to the average. The fact that this was a subject that was of interest to students and still is means most of those attempting to change are already at their physical best.