Extreme immersion into some narrow skill or mode of thought. For instance, doing nothing other than playing chess or tetris or minecraft during all waking time for a month. This seems like it would have big cognitive effects. I'm not sure whether the effects would be good or bad, but it's certainly worth trying.

In my experience, doing stuff like that screws up my cognition in far less than a month -- even after two days of doing the same thing “during all waking time” I start to feel I'm (temporarily) losing my ability to think in a normal way. YMMV.