That’s a nice thread to collect ideas
I would say anything that wires new brain connections:
Learning a new language
Learning an instrument
Dancing
Cooking
Singing
Chess
Using your left hand when you’re right handed
Books that explain specific world concepts
Maths
Programming
Painting
Meditation
Sudoku
Certain board games that require you to answer questions
Discuss, debate
Write
Essentially, do as my nephew does: always ask why. And then ask why again. And again. Think think think, deeper and deeper. (The other day he told me he thinks we can create a quantum level star collision to create a mini black hole, and when we enter it we can use it as a teleporter across time and space. I told him to become a rocket scientist.)
Specific techniques I’m not sure about but what helps is a clean lifestyle, a fun and creative attitude and a playful desire and curiosity. Plus a really good sense of humour;)